Ally: Thining the blood, and vasodilation are two separate issues. Blood "thickness" is dependent on the % composition of platelettes in your blood. Vasodilation is just the expansion of your blood vessel walls. Both can have a similar effect in that they both result in reduced blood pressure.
That said, I agree that you should probably stay away from Hot-Rox pre- and post-surgury just to be safe. Depending on your health condition, any non-surgically induced reduction in blood pressure could be extremely dangerous, (since every surgery decreases blood pressure to some degree...some more than others).
Best of luck during your procedure and recovery!