I encountered one on an unlit bridleway on the way home last night. All in black, no lights. It was on a long straight-ish stretch. I was head-down fighting into a headwind which was pushing heavy rain in front of it, doing a look up every few seconds to see what was ahead.
Course, I didn't see her until she was close enough for my lights to pick her up. But she must have been able to see me from some way away. But any thought of taking some sort of action - audible warning, evasive action, anything - was clearly beyond her; she just continued on the same course, wobbling a bit, and left it to me to sort things out.