My poison ivy. turned purple after swimming is that a good thing?

Poison ivy: skin rash is an allergic response to the poison ivy and that is characterized by dilation of blood vessels in those area. Swimming can increase blood flow to those area and if for some reason there is stasis of blood the area can look purplish. If you have other symptoms such as fever, feeling tired you need to be evaluated for secondary bacterial infection

No, but not bad: The change in color is likely due to change in temperature affecting the vascularity of the inflamed area. It will not have any effect on the healing process.

Usually: It means that it is healing as long as it the area hasn't gotten larger.