Hey, not overanalyzing! I love #vampire physiology, and this is a big topic in that. Fortunately, it’s an easy answer. 

No, dead blood is not bad (as in lethally poisonous) to Ricean vampires. It just tastes like nasty old coffee *spits*

{{Oh Louis, bb, we know, it gets cold so quickly…}}

You’re right that movie!IWTV was not explicit on this, I will explain u a thing based on the movie & canon ;}

The rat blood: is just as drinkable as the whore lady’s blood in the crystal glass later on in the story (the rat is dead, but the lady is still alive, when their blood has been poured into glassware, and in neither scene are the vampires poisoned that way). Blood extracted from a body is not bad in itself, but it cools with the exposure to air, and cold blood tastes bad as I will explain under the cut. When a vampire feeds from the victim directly, there’s no air contact with the blood, and it stays – preferably – hot. More than that, there’s also the entire multi-sensual experience of the act of killing which is way more fun than just the consumption of the nutritional value of the blood. 

Lethal/poisonous blood is not about the blood itself, but is about the moment of death of the victim: What Lestat warns Claudia about in Vampiring: 101 (and he warns Louis in the book, too) is that she must stop drinking before the victim’s heart stops, at least in the beginning, or else the victim could take her down with them in death. That’s more about the soul separating from its body at the moment of death. Older/stronger vampires can keep drinking and slurp the impact of the death down, too.  

Hit the jump for canon stuff, spoilers in there…

In TVL: Lestat goes to Armand in Paris for help after Claudia and Louis try to assassinate him a second time, and Armand throws him in a locked cell with a dead mortal for dinner: 

“Sometime in the dark, I discovered a mortal victim there. But the victim was dead. Cold blood, nauseating blood. The worst kind of feeding, lying on that clammy corpse, sucking up what was left.”

^So clearly dead blood is not bad in the sense of being poisonous, just icky 😛

AR answered the dead blood question at a booksigning ages ago, that dead blood is like “old coffee that’s sat out for awhile. Just distasteful.”

Lestat does say in the movie (and this is probably where the confusion about the supposed lethalness of dead blood comes from, too), “You let me drink *dead* blood?” and it might seem like he means that the deadness of it was the lethally poisonous aspect of it, when in actuality he knows he’s been drugged, it was the absinthe & laudanum combo that drugged him. Still, those drugs are not poisonous to a vampire; he asks to be put in his coffin like a mortal might want to be put to bed, to sleep it off. 

Claudia did it to bring his defenses down so she had a chance at killing him. He couldn’t fend her off in that drugged state.

Who knows why director Neil Jordan didn’t clarify this, and why he had the line 

“You let me drink *dead* blood?” My guess is that he wanted to underscore Claudia’s betrayal, she had made a “peace offering” that was actually a Trojan horse, designed to enter Lestat’s system and weaken him from within. 

Which is really upsetting, especially from a daughter to a father. That moment when she convinces him she wants peace, he looks at her with the most tragic expression, as Amy Nicholson wrote in her book Tom Cruise: Anatomy of an Actor, 

“When Claudia starts her assassination plot by bringing him a human gift, Cruise’s eyes show Lestat’s surprise that someone has finally done something nice for him for the first time in the film… In that moment, we realize that while Lestat is capable of love, he’s never been loved back.”