Blood Red and Chaos Black would make a fine start for an army theme. However, they are both strong and dark colors with some challenges to overcome. It is difficult to highlight right without it looking either pink or orange. (Though it's strong enough to not require highlighting most of the time if you choose) And the black is tricky to highlight since thick or sloppy highlighting will make it look kind of gray. Also note that shading is not possible on a true black base (nothing is darker than black)
There are many good articles on painting in these colors and you should be just fine if you consult them and give it a little forethought.
Try looking at the Craftworld Eldar Codex in the color section of Saim-Hann for an example of one type of red/black color scheme.
Best of luck.