Sudden appearance of very dark blue vein on top of my foot. no pain but some mild tingling. no injury. i do tale dasatinib for cml and am 54.

No problem.: This doesn't sound like a significant problem. If the vein gets hard and painful, it might be a phlebitis, but at that location would be low risk. Discuss with your physician if symptoms worsen.

Keep watch: I would not worry about it unless it does not go away in a few days or gets worse. There is a good Podiatrist in your area if you need it looked at. HVFA in Fishkill; Dr. David Kim.