To correct the misinformation here, let me clarify.
I've noted from my VAST experience with evil biomes, if there's a rain (99.999% blood-rain related) that's red or dark red, it has a severe bug with it that causes some kind of strange 'invisible map wide evil cloud' effect.
Let's say it rains dwarf blood, nothing happens. You look at your dwarves, see the blood on them(contaminant) - but that's it. Suddenly they start dying from rot, with no new rain occurring. You look again, and you'll see 'nefarious soot' (or similar cloud types) on them as well with no reason/cause. I've ONLY encountered this bug with blood rain (red/dark red rain for coloring, other rain that's named like snotty goo doesn't cause this, even if red).
basically anytime you embark in an evil biome with blood rain, you're 95% likely to have a very nasty cloud syndrome that's map wide appear randomly without warning. Some clouds actually do nothing at all and so sometimes, the 'blood rain' wont' do anything - but it's less then 1:20 or so from my experience.
Further to correct meph, blood rain cannot co-exist with clouds (natural appearing clouds on the edge of the map, etc) or vice versa. It's a cloud or rain exclusively - never both for evil biomes, and clouds/rain are also exclusive to the same 'biome' (read the name of the biome on embark screen) - cloud or rain will always be the same in that biome. Rain-cloud syndrome is a bug, which would be quite fun - if clouds weren't like 80% thralling in frequency, removing any of the more 'dangerous-but-not-thralling' fun out of this bug.
tl;Dr
Red blood rain is often bugged with a cloud syndrome that is map-wide and unavoidable. Most likely best to abandon if you see blood rain until this bug is fixed.
