So i have a newly formed blister that i popped and it left a dark pigment and it changes size and it isn't round like a freckle, could this be cancer?

Unlikely: Cancer is unlikely, however, it is not feasible to know what it is without examining the lesion. It if does not heal in a couple of weeks, it would be prudent to see a doctor.

Post inflammatory: Post inflammatory hyperpigmentation is a common and normal response during the healing process. When traumatized skin heals, redness, followed by dark discoloration often occurs. With time, the site will usually resume normal coloration to match the surrounding skin.