I got 30 off an Alpha Bronto, but rarely see those, the Alpha NPC's drop Prime Blood Packs with 50 or 100 in them, can't remember, about 50% of the time though.

bosses wont harvest anything like the dodorex,mega and dodowyvern its been like that for a while. last time i played EC only the alphas dropped Prime blood and i used a normal wyvern to harvest

In case, try to harvest with your pickaxe. Some said, you wont harvest the prime blood with a creature. Also in general pickaxe gives more than your creature.

I also increased the spawnrates of some dinos that give prime blood via the game.ini.

DinoSpawnWeightMultipliers=(DinoNameTag=MegaBronto,SpawnWeightMultiplier=3.0,OverrideSpawnLimitPercentage=true,SpawnLimitPercentage=3.0)
DinoSpawnWeightMultipliers=(DinoNameTag=MegaRex,SpawnWeightMultiplier=3.0,OverrideSpawnLimitPercentage=true,SpawnLimitPercentage=3.0)
DinoSpawnWeightMultipliers=(DinoNameTag=MegaSpino,SpawnWeightMultiplier=3.0,OverrideSpawnLimitPercentage=true,SpawnLimitPercentage=3.0)

Default is 1.0, change that for what you like.

Thanks for the replies. I've found harvesting with a creature to be hit and miss, sometimes it work other times no.
Shame about the dodorex, ape etc not harvesting, seem a waste