This was brought up last night at a casual game.
I played {Blistercoil Weird}
Then next turn I played {Thunderbolt} and put a 1/1 counter on weird and attacked for 2.
My opponent said I can only put a 1/1 counter on him when it is my opponents turn cause It says to untap him
Is this true?
No.
First off, you don't put a +1/+1 counter on blistercoil, it's just a buff until end of turn.
And yes, you can untap him and give him +1/+1 on your opponents turn, effects that make you untap or tap creatures can happen anytime, not just on your turn.
There's +1/+1 until end of turn and there are +1/+1 counters. Until end of turn is what it says. It lasts for the turn, and at the end of whoevers turn it is, the effect ends. +1/+1 counters last indefinately until either the creature dies, or some spell or ability removes them.
Hope this clears it up a bit for you.
He seems pretty good in a deck where all that happens is the casting of spells and then give him unblockable