I'm halfway through discerning the mundane, level 46 character.

I can't collect blood. Looks like when activating a corpse I shoipuld get the option to search or harvest, but I only get the search...

Am I missing something? Do I need some item in my inventory, or am I just bugged?

You can only harvest from 5 races and you only need to do each one once. Once you have harvested from a particular race, you will no longer get the option for that race. If you look at the quest under the pause menu, it will tell you which ones you have left to do.

Thanks, but it's showing them all as needed - I can't harvest any of the races right now...

There is an item which should be in your inventory under the misc tab. It's called "blood extractor" iirc and can't be dropped. You may have to talk to the quest giver again. Tbh though, it's starting to sound like a bug. How long ago did you get the quest? Maybe one of your autosaves is still available.

Aaaah. That'd be it. No extractor (and don't recall being given it...). I'm pretty sure I've got a save. Thanks!

Bah. It's bugged. Even my pre-quest saves don't work. I get an Essence Extractor and still can't get blood.

Anyone know if this is a Main Quest-killer? Or just a way of avoiding the internal agnst over whether to use the levelling exploit or not?

It does NOT affect the main quest in any way. It's a side quest you get at the same time. One of the races is an Orc. Try killing an orc and see if you get the extract blood option. Some of the other races can be hard to recognise. Or try a falmer.

EDIT: It's a Daedric quest.

I have the essence extractor, but get nothing to indicate what I should to to collect blood. can't use the extractor, and get no on screen messages.

not until later in the quest