Hello~I am sorry you are having these issues. Since you have seen all the doctors and the tests are all negative, I would suggest seeing a chiropractor, you perhaps could have some misplaced or pinched vertebrae in your lower back causing this problem. The chiropractor will take some x-rays of the area, study them, and then go over the results with you. If it is something that a chiropractor can help, then he/she will recommend a treatment return, after a few treatments, this situation should clear up and you should feel better.
Hi, it might be as simple as a ganglion cyst at the toe joint - and the stalk of it has been pressing on a nerve all along. Did you see all the specialists *before* the bump was big enough to be noticed? That can explain why they scanned your back but not the toe. The bump probably is a big clue.
However, a ganglion cyst should not make for a dark area... unless maybe your shoes are very tight, and that's causing around the cyst some minor bleeding. Hard pressing might have even caused the cyst in the first ace.