I was told once that closing your eyes you can just use the 'back of your eyelids' as a black mirror to scry on. That doesn't really appeal as much to my artistic nature as my homemade black mirrors I've done with ritual paint do, but it's a good thing to keep in mind for when you're on the go.
Yes, I use this method as it works better for me. Scrying Results aren't the strongest for me but I can use this method with about 70% success compared to lower percentage for other methods but practice makes perfect.
I'm in two minds on scrying whether it is better to specialise in one or two methods or regularly change, but then if you had to scry another way due to outside reasons you could find it hard as you are 'too' used to using certain methods, however the specialisation could be of benefit as the scrying would flow easier.