There are several ways of inducing the kind of blood-related issues which can lead to a stroke, but perhaps the most effective - if you want to term it that - is the use of hydrogen peroxide solution. There’s an article on the mechanism by which H2O2 would cause this to be found here - Hydrogen peroxide-induced stroke: elucidation of the mechanism in vivo - and I suspect that because the chemical degrades to just water and oxygen very rapidly, no trace of it would be found in the body at a post-mortem. Quite how you’d introduce it into someone’s blood stream without them knowing and without evidence of a puncture mark, I don’t know.