Caffeine can increase blood pressure, along with MANY other factors. The two real factors are:
1. ADH production (Anti-diuretic Hormone)
2. Blood vessel constriction
Stimulants routinely constrict arteries, raising the force necessary to move blood through the body. I won't get too detailed on this because the last thing I want to do is write about catecholemine or however the hell you spell it. It doesn't play a huge role here, but if I start in on this, I'll end up writing about nerves, synapses, and the sympathetic nervous system.
Anastas Dimitar Martin