Can you teleport outside? Sure.
Is it safe? So long as there's no rain to smudge your diagram it's basically as safe as doing it indoors. And in Lucien's defense there wasn't any rain at all, right up until the last sylabble of the incantation.
"I could buy a slaughterhouse and launder the blood through that? Or try figuring out a permanently refreshable blood set-up. Assuming that millieays doesn't have access to some sort of bulk waste disposal service?" That last bit is directed at the Bar.
Not exactly; things tend to get out of the way over time but not to the point where it would be convenient to have blood flowing through the halls. Let alone into the lake; there's a giant squid living there and he might object.