I'm close to the darkness, maybe 30 mins walk, but there is no bile drain here - 5/5/5/5 alts have no problem running around my claim.
I set my character to chip ore in my mine then go AFK, come back and I'm at my lean-to with an empty inventory. I was low on phlegm, but there was no cave in or any snakes in the mine (lol) so there was no way for me to get KO'd - but I had. Go back down and still had a little bit of phlegm (like .2), I can see my pile of chipped ore - then I KO again. This time, I'm sure it's bile drain - since I saw it tick me down.
Rest, eat, check enders. Normal 'brightness' is 62.75, outside I'm at 50.10 - normal mine seems to be 25.10, it was at 20.X (can't remember the exact number.) I watched it tick my biles down, (around 35-40 apiece), so I know I'm not crazy. If it weren't for the darkness tracker on Enders, I might have missed it.
Sitting around for a few moments afterward, and everything is back to normal. 62.75 and 25.10 are back again - so what happened? How did the darkness expand onto my claim? Also, note that it retreated very soon after. I know I was KO'd twice and now everything is fine, so it's not something I did - chipping stone will not KO you!