If an opponent gains control of your creature until end of turn, who chooses whether it blocks?

The player who controls a permanent at the time a decision needs to be made is the one who makes that decision. This is established by CR 108.4: 'A card's controller is... the player who put it into play,' but when control has been transferred, the new controller takes over all decisions for that permanent.

During the declare blockers step (CR 509.1), each player declares blockers for creatures they control. Since your opponent now controls your creature until end of turn, it is your opponent who decides whether that creature blocks, and if so, which attacking creature it blocks.

This matters strategically: your opponent might use your creature to block one of your own attackers, or simply choose not to block with it at all. You have no say in the matter while they control it.

Concrete example: You control a 3/3 Bear token. Your opponent casts Act of Treason, gaining control of it until end of turn. On their next turn (which is yours—you are now the attacking player), during the declare blockers step, your opponent decides whether the Bear blocks any of your attacking creatures. You cannot force it to block, nor can you prevent them from blocking with it.

Control effects that last 'until end of turn' fully transfer all in-game decisions to the new controller for the duration, per CR 109.5 and CR 509.1b. You regain control at the beginning of the cleanup step (CR 514.2).