From John: Block+Grab can be countered by Humiliation (as anything can be), OR by Freedom. And yes, a Block+Grab can be followed by anything that Grab sets up, including Choke/Headlock/Powerplay (And Powerplay in turn allows a free basic attack as follow-up if the player has it).
A classic scenario goes like this:
John plays Jab on Renee. Renee plays Block (eliminates Jab damage) + Grab + Powerplay + Hail Mary for a total of 10 points damage back on John. Ouch.