If I summon a creature with the flash rule in my opponent's turn as a response to their attack is it able to block?
Or is it suffering from summoning sickness and can't do anything?
If I summon a creature with the flash rule in my opponent's turn as a response to their attack is it able to block?
Or is it suffering from summoning sickness and can't do anything?
If you flash the creature in during the Declare Attackers step or earlier, you can block with that creature. You can do it after attackers are declared, but it's too late once blockers have been declared.
"Summoning sickness" prevents three things:
Blocking is permitted.
It would have summoning sickness but that doesn't prevent being able to block. Summonning sickness only deals with being able to use abilities with the (un)tap symbol and declaring as attackers. So yes, you can block with it.
Summoning sickness means it can't be tapped. You can still use it to block an attacking creature. Just remember to flash it in at the end of the declare attackers step!