At some point Terezi mentions knowing his blood color and Karkat says something to the effect of "Promise you'll keep it secret until I'm ready to tell them" (so presumably nothing happened that he let anyone else know?) and Terezi says "Sure but I'm pretty sure they already know." Later he refers to it as "the worst-kept secret in all of paradox space." I'm not really of the opinion that anyone was surprised.

Still, it'd be really interesting to know about the exact moment when anyone (or everyone? or everyone in turns?) figured it out. There seems to have been a widespread agreement to let it be an elephant in the room but I wonder how that was agreed on, or how it looked like it without really being the case.

I always chalked it up to meteors. I mean I seem to remember references to expecting her to die soon--presumably of old age, but old age doesn't make people scream, so I assumed there was some meteor-induced pain involved.