Blood blister or possible cancer? (Photo)

It is hard to tell for sure from the photo.  It needs to be examined with dermoscopy (basically a polarized light and magnifier tool).  If it is simply a hemorrhagic blister that will be pretty obvious with the dermatoscope.  A benign nevus or mole would be easily diagnosed as well.  If it looks suspicious with the dermoscope, then it should be excised and examined by the pathologist.  Don't get it cut out without it first be examined with a dermascope!!!!  It may save you a procedure.
Best of luck to you,
Dr. Amy
 

Unfortunately, it is tough to tell without a good exam and examination under the microscope. Most likely, you irritated this area as it appears to be in the bra line. However, the best way to make sure is to see a specialist who will cut it out and send it to the lab. Note: a "shave" biopsy should not be done and it should either have a piece cut out from it (incisional biopsy) or the whole thing should be cut out (excisional biopsy).

With any lesion on the skin that has changed in anyway, I would always recommend getting it checked out by a dermatologist or Plastic Surgeon.  The vast majority will end up being benign but I would always want to be sure.