It blocks most of the alkaloids in Kratom, but not all. Kratom is complicated and unique, it acts primarily on opiate receptors but also acts as a stimulant. The majority of the receptors that Kratom would act on are blocked my Suboxone, but not all of them. I've taken Kratom and Suboxone together, but only when I was on a very small amount of Suboxone. Anything more than a few mg of Bupe and I don't even feel any need or desire to take Kratom --- it's not even a thought in my mind.
I have noticed, however, that you can take Suboxone very shortly after taking Kratom (a few hours later, if not even sooner) without experiencing any precipitated withdrawals like you would have from standard opiates.