In fact, by the time forensics gets to a murder scene, it's pretty remarkable if there's even a body there to examine. The cops don't want to deal with corpses. Well, nobody really wants to deal with corpses (if you get too excited about dead bodies they tend to look at you funny). But the police especially don't want the body around, so they usually call an ambulance to take the victim away before he inevitably shits his pants.
Then there's the actual collection of evidence. We don't just shine a black light around all willy-nilly and swab a computer keyboard or two. No, we collect everything in the room, if not everything in the house. And when said house looks like this, it can get downright tedious (not to mention smelly):
Consider yourself lucky for never having to ask, "Are mummified cats considered evidence?"
Oh, and speaking of black lights, I'm about to ruin every single detective show on TV for you right now. Ready? Here we go: Black lights don't detect blood. Well, they do, but not in the way TV depicts it. In college we had a session where my instructor took several fabrics and smeared various bodily fluids on them (forensics courses are ... interesting). Then she took us into a dark room and had us shine black lights on them, and that's when we discovered that blood absorbs UV light and appears as black splotches. So what's that bright, glowing fluid all the TV black lights are revealing? The answer, of course, is semen. Every single episode of CSI: Miami features David Caruso standing in a room absolutely drenched in man-batter.