What caused blisters after sclerotherapy?

The most common cause of what you are describing is irritation from the tape or bandage.  I have seen this occur if an ace bandage slides/rubs over an area and also from tape irritating the skin.  It would be very unlikely that the sclerosing solution caused this problem.  A topical cream or ointment such as silvadene or bactracin would help.  If the veins were injected, then they should resolve over time.

As a vein specialist who does a lot of sclerotherapy in my office, I have unfortunately seen this type of reaction before.  Fortunately it rarely occurs and tends to happen in my patients who are very active after sclerotherapy treatments.  It results from friction of the skin against the tape and compression wrapping.  Much like wearing a misfitted pair of shoes that causes blisters, friction from the compression rubs against the skin to cause blisters after sclerotherapy, especially if you do a lot of movement on the legs.  It is usually not a true allergy to the tape or adhesive (that usually appears as a rectangular mark exactly the shape of the tape).  And it is unlikely due to the actual injections themselves.  

The good news is that this should resolve without leaving any marks, but be sure to take good care of the blisters (keep it moist with vaseline).  And if the veins were properly treated, they should go away as well!

Best,

Dr. Mann