Yeah, you’d expect whatever was put in his drink to exist throughout his system. You know how you can smoke pot and have a form of it show up in your pee (a metabolized byproduct, not the drug itself)? It’s like that, but it also (in both cases) applies to all of the blood in your body. What are they going to do? Pump all the blood out of his body and replace it? That’s extremely difficult if not functionally impossible and, assuming it’s possible, would both cause an external wound and probably fuck up his circulatory system structurally. It’s also just logistically not possible unless you mean that the coroner himself did the cover-up or someone else involved in the legal custody of the corpse. And that’s probably the easiest part, because the evidence would also be embedded in other fluids, like those in the eye, tissues throughout the body, and maybe even hair.

If this person was poisoned and a second coroner is being hired under suspicion of the first one being dishonest, there is no way they wouldn’t be able to discover this fact even if truly extensive and absurd measures were taken to try to hide it.