It's a thorn or a bit of a splinter imbedded in your leg. I would cover it with a large telfa pad well covered with an antibiotic ointment....and see if it improves in 24 hours. A spider bite has an area of necrosing tissue, but not that black spot. If it were on your hand I would say soak it in warm epsom salts solution.
I had a similar thing on my hand, went to the Dr to have it dug out.....and he looked at it and essentially told me to go "soak your hand"....and I did and it eventually came out.