I've had Morrowind for years now, and still play it often. Also have tribunal though never really got much into playing it.

Now I got bloodmoon and want to install it... and running setup from the game CD returns this:

You must have Morrowind installed to install Blood Moon.

???

Anyone got a workaround for this?

I've had a similar experience when attempting to "repair" the Morrowind installation from the setup program. It seems that if you select the "Repair" option, it does something to the registry keys (well, that's what I suspect) such that the expansions don't recognise that MW is there!

Eventually, I had to uninstall, then reinstall MW before I could get Tribunal running. I hate to say this, but you might have to do just that. Sorry.