Over on BrainFuel, there are some tips for those looking to design a website which of course applies to creating a design for a blog.
He mentions things like typography, content density, dealing with the details, and whitespace.
I, personally, have trouble with typography. It is not my strong suit, and I know it, but BrainFuel recommends making sure that the type that you chose is not being made to look better thanks to settings like strong, sharp, and whatnot, as your HTML presentation of such text may not look as nice, so test it out in web browsers before you call it a day.
Check out BrainFuel for more on his other tips.