I have no direct knowledge of it's use for bp reduction - however, since caffeine is also a diuretic, given the structural similarities between it and theobromine, i suppose theobromine could also be a diuretic. Again, i have no direct knowledge of this application personally, and while the wiki may be right, i've seen far too many things on it that are simply inaccurate for me to trust it as a good source 8-) (i've not looked at it in ages, but the chocolate wiki, for example, was (is?) riddled with inaccuracies).
I would unequivocally NOT make a 'tea' from the shells and drink that. You're going to get all sorts of unwanted nasties that you absolutely do not want to ingest. The entire point of crystallization is purification. At some point, you're going to need a separation step to help discriminate the theobromine from the other water soluble stuff - if you went straight to xtallization, sure you'd xtallize theobromine, but also precipitate out lots of other stuff.