What Causes Dark Colored Blisters On The Thumb?

Hello, 2 days ago I had a small line of blisters appear on the left side if my left thumb. Now, two days later, it has developed into a blister about the size of a dime. It is deep purple and is not painful or itchy, as it was two days ago. Do you have any idea about what this could be or how I should treat this?
Thanks for any information you may provide,
Kitty Messner