I've been trying an experiment lately. It seems similar to what others have done in the past, but I've not seen anyone do this specifically.

When I'm going to sleep, I stare into the darkness you see when you close your eyes, and I try to get a "wall" to materialise. It's quite a passive action - I don't really try really hard to imagine it, but I try to let it happen. Sometimes it does (sort of) and sometimes it doesn't, but I don't force it. I don't try to dictate where the wall will appear - sometimes it seems like it's far away, sometimes it seems like it's a couple of inches from my eyes.

In dreams I'm often seeing surfaces that resemble the weird grey/black blobbiness. It's always been a surface so far. This may act as a dreamsign, or there may be some other way it can help to achieve lucidity. I will keep doing this exercise and see where it goes. So far I haven't used it with WBTB because I only do WBTB if I wake naturally and recently I've been sleeping right through.