A purple or blue/black mark on a toe nail is most likely to be caused by a fungal infection (so it's good they are checking your nail clippings) or trauma (you've knocked the toe etc). Having the same marks on more than one toe, especially on both feet, leads more towards trauma caused by ill fitting or new footwear. I've had this happen after getting new walking shoes which were a tight fit on the toes. It takes a long time for bruises/marks to grow out of the nail (6 to 12 months). It's very, very unlikely to be melanoma of the nail (subungual melanoma). This usually presents as a black line from the nail bed to the tip of the nail (although it can sometimes be a black spot). It only usually occurs in one nail so, having these marks on 3 nails on different feet makes it even less likely.

I hope this helps,

Angie (Stage 3 melanoma patient since 2009)