Damp corner in living room & blocked airbrick

Jinxii

Hello!

We've owned our home for two years now, and I've recently noticed a patch of damp on the back wall (next to a radiator and below a window) spreading. On the other side of the wall I've noticed an airbrick, but because the previous owners were very much bumbling DIYers and painted the ENTIRE Victorian terrace house blue, they seem to have painted over the top of it. Some holes are totally blocked with paint but most of them are partially covered. The worst of the damp is directly on the other side of the airbrick, so I assume this is the problem...but also the airbrick doesn't come through to the other side of the wall, so I'm not sure what unblocking it will do? I've read a bit online and apparently it may be for the underfloor cavity, but then I'm not sure how it's making my wall damp. Any suggestions? (In layman's terms, please!)