No window treatments are going to give "total darkness" Anyone who sells you something and says it will is a liar. Even the best blackout shades installed with side channels will block (maybe) 90% of the light coming in a window. Call a local professional to come out to your home with samples to look at. Best options will probably be a double-cell honeycomb shade or a roller shade.

I don't know what to do with my bay window

I have two of these windows both identical in my living room. 1.6m width by 1.2m high. All four windows open independently inwards. I’m struggling with what the best window treatment would be. They also both have a radiator underneath which can’t be blocked and my L-shaped sofa is in front of both also.

I would love a window seat so we are considering replacing our current window with a square bay window to allow for a window seat. Is this a big job? Would it be expensive? Or would anyone have another suggestion? Thanks