THIS season's hot holiday color is none other than black. Depressing? Not at all. The effect is more black tie than black magic at the Santa Monica housewares emporium Luxe Haus, which trimmed two black trees with hand-blown German ornaments in white, smoky gray and ebony. (See Page F1.) "It's very simple and very elegant," says Lou Peixoto, who coordinates displays for the Montana Avenue store. "Yet there is an edge."