Oh, I don't think it is at all like Viagra...please don't assume that.
To work like Viagra a substance has to dilate (or increase nitric oxide), and
if anything.. huge use of B12 would make blood thicker, and impair that function IMO. But for cold sports like the Winter Olympics, those cross country, or
endurance events, need huge oxygen carrying capablity in the blood. That is why ProCrit was used in doping and finally banned. What can happen to those who try it, is that the blood can become thicker, and sticky and lead to blood clots or stroke. Since most of them are young, it is less likely. But still a risk.
Many of those athletes train at high altitudes too, to increase red blood cell concentration....there are lots of tricks...but I don't think many of them affect the below the belt functions.

Now if anyone is WORRIED about blood thickening.... older adults can be typically low in blood cell volume. As we age we develop
mild anemias. My doctor was ASTOUNDED by my last blood work. She expected this since I am over 60 now. I didn't even get
anemic when I was pregnant and had SURGERY and huge blood losses. So I am a special case I guess.

When we talk about blood doping or extreme endurance that does not include US here (I hope).