Hi CCL,
I often freeze capsicum; there is no way I'm wasting it!
I simply dice it (quite small, raw, unblanched) and chuck it in a freezer bag for later use. I have never detected any bitterness from doing so. The only thing is that, having a high water content, it becomes mushy on defrosting.
So don't count on using it in things like a Jalfrezi where you want the capsicum to retain some "crunch"...cos it won't!

But it's fine to use in things like a base and if adding at the start of a curry. It would also be fine as part of a tarka (provided you don't want it to be crunchy).
Hope this helps.