It's kind of complicated, unfortunately.
You start with a list of vectors in "CIE colour space", which define the colour associated with various wavelengths across the visible range.
Then you integrate out the Planck equation for the black body spectrum, and figure out what proportion of the total output in the visual spectrum is occurring in the wavebands around each of the wavelengths for which you have a colour vector. Multiply each colour vector by the corresponding proportion, and then sum all the resulting vectors to find the approximate CIE colour space coordinates corresponding to the black body spectrum.
Then you need to convert the CIE coordinates to RGB coordinates for your computer monitor - this involves some vector algebra, and the conversion depends on the CIE coordinates of your monitor's phosphors and its white point.
Unfortunately, my sources for the detailed workings are various books I've collected over the years, all now out of print, and I haven't been able to find a decent web resource I could point you at.
But if you have access to MathCAD, I'll happily e-mail you the MathCAD formula sheet I've built to do various colour and brightness calculations for stars, using the blackbody approximation.
Alternatively, you might just care to let someone else do the whole job for you:
http://www.vendian.org/mncharity/dir3/blackbody/ 
Grant
