I think reactive blocks aren't always a "No Go" situation: If a Wizard has the forthought to make an enchanted item with one, it counts as a reactive block, and I believe with a bit of Thaumaturgy and time, a pretty good block spell could be placed on oneself or in a potion, Although it would be a One-shot deal: wait until the turn before you think the big boom is going to come, and set off the Wall of Protection.
On-The-Fly, attack-is-coming-now-so-cast-that-spell reactive blocks, I don't think would be a good thing to allow, without the character at least having Initiative and holding his action to see wht's coming in.
As for allowing Counterspell Defenses, Remember Harry & Billy's Margin Notes on YS253:
I don’t think counterspells can be done reactively, due to the assessment requirement.
If you have a held action, You might consider that
proactive counterspelling, much like the above block, and allow it.