One of the arguments I've heard about blueprint css is that it lets you think of your layout in terms of a grid rather than in terms of floats. This seems like a big advantage to me because I always get confused about where my floats are going to end up - sometimes a float will unexpectedly drop down below some other floats and I have a hard time figuring out how to reposition everything. Does this make blueprint css a good choice for me - or should I just put the time in to learn how to use floats properly and do all of my layouts manually?
Yes, absolutely it lets you use a grid - see this example page, which makes the grid visible by striping the background according to the grid columns.
(But I have to say, a little time spent learning how CSS layout really works would pay big dividends - better to understand it than to give up and try to work around it. And you'll then have the option of creating flexible layouts that work well on all sorts of different devices.)