I could make this mono-blue but I feel like it's power level is super high. It makes the hardest counterspells into mana leaks.

Situational, but a nice sideboard card. I think it may need "by an effect an opponent controls" to avoid Shennanigans.

It's also kinda a blunt force thing. Like "Oh crap, counterspell decks are way too good, find a way to nerf them, yesterday!". Are they too good? Can you find a subtler way to reduce them in power?

This is lame compared to stuff like Vexing Shusher, Cavern of Souls or even Leyline of Lifeforce. This could easily say "spells you control can't be countered" and still it wouldn't be that amazing. Turning straight-up counterspells into Mana Leaks does basically nothing in non-EDH match ups. Would you like drop this and then wait till you can pay extra for your spells? That's inane.

Also, anti-counter tech is generally concentrated so that can stop creature spell countering while can block noncreature spell countering. In any case it's not in though pretty much a spell of any color can have the text of "~ can't be countered."

A less direct way to hose draw-go decks is use effects that force you to "play on your own turn" such as Dosan the Falling Leaf and (a more recent) Dragonlord Dromoka. That too is a effect.

EDIT: Actually that effect might have shifted to (Grand Abolisher) since is the color with the most "rule setting" cards such as Rule of Law.

Well, if it's not op then I think it's ok. Sure, this won't see modern play but it could be seen in block constructed

Blue is tertiary in "can't be countered". Black and white don't get it at all except on gold cards in combination with R/G/U. But apart from that, I agree with Tahazzar. Even if I had a deck that was suffering horribly from counterspells, this would be painful to play. If it just flat-out said "Spells you control can't be countered by spells or abilities" then that's one thing, but still charging per counterspell feels really depressing. Compare Vexing Shusher which is cheaper, can attack/block, and only charges 1 mana to protect a spell.

I would, at minimum, make it so Insistence can't be countered.

Also, it saddens me to say, but 'block constructed' isn't a thing anymore. Or to be more precise, it's not a format officially supported for tournaments, ever since blocks decreased in size. I still think it's a fine card for casual. But I also think that Horror of Horrors is a fine card for casual, so... you know... opinions.

": target spell you control can't be countered by spells or abilities."?

I'm still not really seeing the need for this. Especially since it's taking up a precious precious uncommon slot.

It works, it's meh. Some of the suggested changes make it less meh - but still horribly horribly niche.