Has the tree been injured, even struck by lighting perhaps? If not, can you find a spolt or crack in the trunk? Has there been a change in the grade around the tree or something toxic sprayed in the area? Could something a close neighbor has done recently impacted it?

That should be "split" not "spolt"

Is this a young tree? Could it have freeze damage. There is a Mexican plum on part of my property that is almost a hundred years old and my mom has raised them from seed and I have never seen one with "blisters". I hope someone can help you. I love Mexican plums. Jelly made from the plums is wonderful.