I control a Nyxborn Shieldmate enchanted with a bestowed Nyxborn Shieldmate, and no other creatures. My opponent casts Barter in Blood. Does the second Shieldmate become a creature “in time” for Barter to force me to sacrifice it?

No.

Barter in Blood has one action (one instance of the verb “sacrifice”). The way it works is:
- the active player chooses what they're going to sacrifice,
- the nonactive player chooses what they're going to sacrifice,
- the chosen creatures are sacrificed simultaneously.

When the choice is made only one of the Shieldmates is a creature, the other, bestowed one isn't a creature and can't be chosen.

Daniel Kitachewsky
L3, Paris, France
Rules NetRep