Temples - General
There are a variety of temples in the world and each functions pretty much like all the rest, though they are slightly different and often members of one order wont even speak to members of another (and who says this game isnt like real life). In the original version of the game it was possible to join all of the temples (which led to some interesting reputation problems). One of the earlier patches removed this ability. You are now restricted to one temple (though you can get kicked out and join another if you want to go through the hassle).
On the topic of blessings, I have not found much help. Each temple supposedly offers blessings, but I have not been able to find anyone at any temple who dispenses these blessings. They will all take your money, but if Ive received a blessing, there is no indication that it has been done.
All temples will heal the wounds of their members for free once you reach a certain rank in the temple. All you have to do is talk to one of the people in the temple and it will be done automatically. This healing will also take care of any damaged attributes or diseases.
All temples will cure diseases -- its 250 GP per disease, but subject to your mercantile skill, so you might pay a little more or a little less. The cure is always effective. There is supposed to be a chance of failure for lycanthropy and vampirism, but theyve always cured me as long as I got there within the three-day limit.
All temples accept donations. Donations can increase your reputation with that temple, but generally do not take the place of completing quests. In other words, theyll take your money, but you cant buy rank in the temples. There is an exception to this. Donations to Zenithar increase your rep with the temple, so it may be possible to "buy" rank with the Resolution of Zen.
All temples will offer training to non-members, but their prices are exhorbitant. If you don't mind paying the price, you can train. A words of caution, though. If you take damage in Holy Places, be prepared for a serious owie if you train at a temple.
Temple quests are pretty standard. Youre usually sent out to kill the heretic, retrieve the drunk priest, exorcise the demon, check out the story of a vision or something along those lines. Temple quests typically involve dungeon crawling, but there are a several quests that dont. Successful completion of certain Temple quests can increase your rep with the general populace.
Your rank in the temple affects your reputation in the regions where that temple is a major religion. Check the overland map and look at the temples in the region to see which god(ess) the province follows. There are a couple of the smaller provinces (Orsinium is a wonderful example) where it's not quite clear what the dominant religion is.