There are several apps and plugins made be freeware developers (****, even some firefox extensions) that will blend your playlist based on key, tempo, etc... but they work about as well as Genius (which is to say "not very well at all, Computers aren't very good DJs). Since Genius is already there (unless you need to update iTunes), use that. It's a pain in the ***, but so are the rest of them.
Otherwise, iTunes regular old shuffle feature actually does a lot of that for you (you can adjust it in the references menu), so you may just be better off using that. If you want a professional (DJ-quality) mix, you'll pretty much have to make it yourself ahead of time.