So basically, some time after Gary's power-up, he comes to Whitney for help trying to master manipulation of blood, something he should theoretically have the power to do but which he is struggling with. He figured that while Ruby would be more suited for the task abilities-wise and may even have better contacts for resources, she'd only be able to do so with delay for the resources and with extreme difficulty teaching-wise given her fighting style.
In contrast, by technical skill and wisdom/experience, Whitney is who he bargains would be his best bet for being a good teacher for the skill--as he turns out, he's quite right, because Whitney does in fact have experience with the subject. (This serves as the second time Whitney is a teacher, since this is after Whitney powers up D.D. the first time, given that D.D. is powered up during the arc that Gary gets his power-up.)
Whitney explains to him that Blood is a proper element in of itself--it may not be one of the Primary Eight elements, but it's one of THE largest elements outside the prime eight. It just so happens to be a rather significant source of magical power, near-universal across all sorts of magic, holding all sort of symbolic power. It's fundamentally featured in nearly every style of magic, in large part because blood is a REALLY big thing in the world of magic, almost as big as the concept of yin and yang.
She goes on a bit of a tangent, explaining that the most prominent users of Blood, the Blood Masters, are like inverted vampires, using their blood magic to fundamentally alter the nature on reality, sometimes on rather large scales. While far from the only specialists in using blood magic, Blood Masters are called Blood Masters for good reason, having full control over their blood. (This, incidentally, gives them vastly-increased lifespans, though does not grant them immortality. The greatest grandmasters lived for about 800 years, ten times the normal human lifespan, before succumbing to magical fatigue.)
They are the absolute authority on the subject, inspiring all other sources that don't predate them. While various wars throughout the centuries (in particular, a rather devastating one akin to the war between the seven races, only in this case being a war between several different unique-element-imbued clans) left them virtually extinct at a point in history, they've been slowly rebounding thanks to surviving with the help of vampires.
Vampires find their blood extra-sweet, and since blood masters naturally produce an overabundance of it (they literally feel compelled to bleed every day if they're powerful enough), the symbiotic relationship was only natural. While one of the seven great blood master clans was left extinct with all records of its existence wiped out and no living people having personally seen their members in action, the other six were preserved.
(For the curious: the first of the seven great blood master clans was founded by the founder of the blood masters. He was a baboon-spear-wielder, and his style of combat dealt with channeling his blood through the spear; he passed this on to all of his soldiers.
His rival used a similar spear-based style, but had the spear be heavier-built: thicker wood with a narrower hollow center, as to allow for more powerful and precise blasts, at the cost of speed and dexterity. Said rival eventually joined him, being one of the founding council members and leader of the second clan.
The third of the great blood grandmasters and founder of the third clan is the one records have been lost. Read: I forgot who he was and what he did. He was important, but I don't remember how. It was his presence that made the blood masters really become unified as a force, though.
The fourth style was a sword-and-shield style founded by the right-hand-man of the first grandmaster, whose powers were originally latent until later activated in the midst of the battlefield. He sought after and recruited similarly-latent blood masters, effectively doubling the size of the order, and went from the Normal of the group to effectively tied for second-most-relevant.
The fifth style was formed from a rounded dagger, with blood being used to turn it into a whip. While difficult to master, this style of combat was one of the most devastating in battle, basically being the elite clan of the blood masters.The sixth style was formed from a grandmaster using a drill, who emphasized redirection: catching an attack, and then spinning it back to spiral back towards the attacker, counter-attacking with blood backing up the original attack.
The seventh and final style was created by the man who ultimately ended up saving the blood masters from extinction: using a combination of bow-and-sword, he was the one who first bonded to a vampire, via his friend...Victor Zu, pioneering their ultimate relationship. He was a prodigy in spears, katanas, longswords, shields, and bows, but for the sake of creating his own style, he chose to focus on the bow and using the longsword as a secondary defense weapon--I think the katana might've been the weapon of the third style, not sure.
Butyeah, that's the seven styles of blood masters, six of which still exist in the Rubyverse.)
Anyway, the point of this Whitney brings up is that she had the pleasure of working with a vampire-blood master duo (the blood master being a descendant of the seventh and using his style, but since the child of a blood master isn't necessarily as strong as the parent, he's the first blood master of his family since his ancestor to obtain the power level of a grandmaster), one Ruby wouldn't be aware of because the duo doesn't stick around in one place for too long.
Said blood master explained that Blood has loose ties (really loose in some cases) to seven of the eight primary elements, with her element of ice ironically being the only element not having those involvements. Blood gets oxygenated to turn red, thus, it has a relationship to air. (Very weakly, though.) It is technically tied to things like iron, which have strong ties to earth, so it does possess a weak tie to the element of earth. It has something called plasma, and is symbolically the source of energy for the body, thus, it has moderate ties to the element of energy.
Similarly, blood has some rather strong ties symbolically to life, and life is generally something associated with the light element, making a strong connection there. While not quite as strong, equally as important is the opposite, noting that blood has a dark side, representing violence, destruction, and the occult: because it is associated with the forbidden, it has ties to darkness.
Then, it has very strong ties to passion, just like fire, which it also shares colors with. But strongest of all is, in fact, water, thanks to blood primarily being made up of it, so Gary's right that he can control it...he just needs to recognize the presence of the other elements there and help 'persuade' them so to speak to listen to him.
She recommends a day of training with each Rider, emphasizing repeat sessions with Ruby, along with training side-by-side with a vampire: Dale being an obvious choice, with other vampires from any resources Ruby can offer him freely. In the mean time, she would go schedule an appointment with the blood master while he trains.
When they meet, they run through the process, and while Gary can't master everything--his element is Water, not blood which is made up of multiple elements--he does grasp a lot of magical-based moves, among them being use of his own blood, along with temporarily restraining or even controlling the movements of an opponent. (Given the tangent that they're not horrendously stronger than a human would be, in which case his power would be useless.) He's warned not to rely on the tactic, especially given its questionable morality, and he agrees.