Occasionally reduces spells' MP cost.
Technically speaking, Conserve MP might
affect the initial summoning of an Avatar since you are actually casting a spell - although I've never noticed it kicking in, and the MP saved would be somewhat insignificant anyway. Bloodpacts however are pet commands, not spells.
Also, it´s description on an item would say "Avatar: Conserve MP+x".
It'd probably be more along the lines of "Bloodpact: Conserve MP+x".