The actual reason as given by one of the creators was that they were trying to diversify their characters by adding a black character when the vast, vast majority of characters at the time were white. It just happened that this character was the theme they were working on at the time.
It fits his theme visually, since he can be in the sun (Daywalker) and the pasty white vamps cannot. But that is just coincidence. Since the vast majority of superheroes still lean towards white due to keeping a lot of legacy characters around, it is important for Blade to stay black because he is part of an under represented race/ethnicity group.
All that said, Santa being white and Blade being black is a meaningless comparison. Santa brings gifts to all children and having multiple representations that fit different groups is a positive. Changing Blade erases an early example of a non-white superhero, which is a negative.