I can honestly brag that I stopped to pee on the climb to the finish, and it was good volume, clear color. Wow, the things we find impressive. I might add that rehydrating was the key, but I also used Karl King's electrolyte capsules during the run and feel confident they helped balance all that water consumption with the hard running the first and last 20 miles. If this was the first incidence of blood that you ever noticed, I'd get it looked at if the color doesn't return to normal after about 24 hours. This is assuming that you are continuing to produce urine.
Urine volume is also an issue.
If there is little of it, you might have been dehydrated to the extent that the bladder walls were rubbing together. This could lead to the blood. Dehydrating to the point that the kidneys aren't producing urine is not a good sign. These are important things to note. You should continue to drink after an event to keep the kidneys operating normally.