It could be challenging to pull off. A lot of blue & black's strengths at common are reactive cards (counters, removal, bounce, search, etc). You would probably have to make the deck very creature focused to ensure you don't wiff when resolving the cascade trigger.
It's definitely a build-around-me commander which is a huge plus in my book. Since your commander has cascade you'll want to be casting it as many times as possible. To avoid having to pay an insurmountable amount of commander tax, bouncing and recasting seems like the way to go. Cavern Harpy seems excellent and Shrieking Drake and Dream Stalker could be OK here, too. There are also things like Curfew, Call to Heel, and Aether Tradewinds. There's also Stormscape Familiar and Nightscape Familiar to help facilitate absurd turns where you cast Kathari multiple times. Shakespeare's definitely right about the reactive cards, though. I don't think this deck can run much if any countermagic. Another thing to consider is that to maximize the value of cascading you'll want to stuff the deck full of 3 cmc cards.
All you have to do is throw in a few counters that are 4 cmc to make sure they aren't hit with cascade. 4 cmc isn't the best option for counters, but a few is better than none. Another option is to create a very specific toolbox of cards at 3 cmc or less to make sure you hit something good. Instead of stocking a BUNCH of 3cmc or less cards and hitting something completely random, you can target a certain type of card at 1-3 cmc and you will have better chances of hitting that. For example, you can run some utility creatures and removal spells at below 4 mana. Then everytime you case your commander you can know you will either get a useful creature, or a removal for that problematic threat.
All you have to do is throw in a few counters that are 4 cmc to make sure they aren't hit with cascade. 4 cmc isn't the best option for counters, but a few is better than none.
What do you guys think about this as a pauper commander? i think it could be very useful, but cascade can sometimes be a dud.
This is a good point. Traumatic Visions and Lost in the Mist come to mind as playables.