[BlazerLT]

I don't deem that backfire. I have heard perfectly tuned engines pop like that when the rpms are falling.

It could be anything really including a little water in the muffler.

Try drilling a tiny hole in the bottom rear of your muffler at the lowest point below where the tailpipe attaches to the muffler.

This will remove any water in the muffler and also extend the life of it as well.