Its not a temperature thing, its a color thing. I don't think it would be harmful to the corals (but you may be getting into the UV spectrum there so I am not sure) but I don't know how light that blue would be at actually growing corals
20,000+ is a good supplement for other color temperatures but you are probably going to see a slow down or a complete halt in growth having that as your only bulb. You need more daylight.