I am just guessing, but very dark blood probably indicates that yu cut the liver. It'll be a miracle if the deer survives a lliver hit.

Don't short change a good blood tracking dog. I have one ... rain, muck, us trampling around, etc. ... not an issue. If there is any blood, even miniscule, a good blood trailer should be able to keep the track going. I'd look very hard along the line on which he came in. Lots of time, a wounded deer will eventually head back from where it came. For sure you'll need to widen yuor visual search area and be very methodical. Good luck !