Can a flashed creature block?
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:


Declaring the sick creature as an attacker.
Activating abilities of a sick creature with the tap symbol (aka {T}) in their cost.
Activating abilities of a sick creature with the untap symbol (aka {Q}) in their cost.


Blocking is permitted.