The purple blobs are ionized gas, in this case a form of plasma called "Corona Discharge". I cannot seem to find any information on how much should be visible on insulators, though what I have found would indicate that "very little" is expected on this type of insulating support.
Any time you are near high voltage wires and hear the crackling, buzzing sound typical of power lines, corona discharge is usually part of the sound generation. I never bothered looking to see where it was specifically being generated before, and was surprised that essentially the only stuff I could see was on the supporting insulators.
Corona discharge, in addition to eventually leading to the failure of insulators (hmm - I recall I live in PG&E territory - wonder if that's relevant), is the way power lines make ground level ozone and nitrogen oxides, neither of which are good for people.
It goes to show, keep your eyes open and you can see the unexpected in ordinary places!