Steve, please search on "toe discoloration hydrea". I'm postting an article that discusses and has pictures of hands instead of feet but your description seems to apply anyway. There may be a match for yours I haven't found. Why your still using Hydrea with a MF diagnosis is a puzzel. Was that diagnosis made from a bone marrow biopsy? Anyway, Hydrea is known to cause leg ulcers so contact your doctor for instructions.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2885649/