When Louis is first turned Lestat and louis are at the dining table. Lestat kills a rat and squeezes the blood into a glass for Louis to drink. When he drinks it and sets the glass down there is blood residue in the glass. In the panned back shot of them and the table the glass is clean.
Take it a step farther... I think it is reasonable to assume that the rat is dead when it's blood is drunk, and we see what drinking dead blood does later in the movie
If you want to be technical about drinking dead blood, it is not healthy for a vampire to drink blood from a body once the heart has stopped beating. But the rat's blood was drained when its heart was beating. Otherwise, it would not have dripped out as easily as it did. The rat's heart stopped beating after being drained. Therefore, the blood was not, "dead blood."
I'm going to get even MORE technical.
Actually, even if it's NOT (here we go again) in the BOOK it IS possible, and Lestat did drink it from a wineglass during the scene with the hooker in the book as well. Nothing happened to him.
Even if it doesn't make sense, I asssume the blood drinking rule only applies to when the blood is in the body.