[Micah]

Blood magic has an ongoing cost in mage time and lost income, and scales with the number of provinces you own, along with gold and gem income. You still give up something for those slaves (3000 slaves in a turn drains an entire capital province's worth of population from your empire every month, on top of the mage time and unrest costs). Blood is really damn good, but it's not really put at an abnormal advantage just because of a large map or a long game (I think it's slightly disadvantaged by a long game because of the population drain, actually). If you want to debate that Blood is unbalanced in general that's a different point.

Clams do not have an ongoing cost aside from needing warm bodies, and scale with game LENGTH. A clam you make on turn 10 will have pumped out 90 pearls by turn 100. I can't think of many other amazingly good uses for water gems, either, so it's not like you give up an opportunity for kickass summons or item forging when you invest in a clam instead (the water queens are amazing, but since there are only 3 of them between 60 nations they're not going to make much of a dent on overall gem consumption)