Can you block a creature thats not attacking you
Can you block a creature thats not attacking you
No.
Fluttershy is best pony.
Why not?
All the rules for the declare blockers step state that only the defending player declares blockers.509.1. First, the defending player declares blockers.
Of course, some multiplayer groups may have house rules (you can block for your allies in team games, you can form alliances and block for them in exchange for something, etc), so make sure its all cleared up before you start.
Fluttershy is best pony.