Right pls don't get too worked up about the numbers, just try to understand the principle:
That still doesn't explain why Kain thinks blood magic wouldn't make an innately stronger mage much stronger.
It would, I just said not a lot, unless many blood vessels are involved. Imagine that a mage has 100 mana, but if he cuts himself he can draw 500 more mana from his blood. Now imagine he has 10 people to sacrifice, so he can draw 5000 more mana from those bodies. 5000 vs 100, that's a freaking huge power-spike. Of course I believe that a mage has to be somewhat powerful on their own as well to draw power from blood, I believe that logically some mana has to be used to establish the link between the blood and the caster. So say 10 mana for every 500 blood mana, so the mage wouldn't be able to go beyond 5000, unless he had a bigger natural reserve of say 150, then his blood limit would be say 7500 etc, etc.
Now imagine that a mage is a ridiculously powerful mage, and has a natural mana supply of 10000. And from his blood he can draw 500 more.
10000 vs 10500, not a big difference eh? Unless of course again there are hundreds of slaves around to sacrifice.
You are right that potentially a powerful blood mage could be infinitely more powerful, but that infinite power comes from from the supply of blood that they have access to.
I was talking to TKS basically about magister vs magister, but realistically, what magister is going to sacrifice 1000+ fine slaves in every fight? And if not, and if they are powerful, they don't even really need to.