Is it a blood spot or does it look more like an irritation? If you can post a photo it would be pretty helpful. This "Blood spot" is likely a bruise, and it shouldn't cause any worry. What you are describing could also just be an irritation caused by the cut. It's probably a good idea to test your water for ammonia because ammonia can hinder the healing process of an injury, causing this spot you are describing. It's probably a good idea to do a water change too just to be safe. Ammonia burns are pretty easily healed after a couple water changes.
If it actually is a blood spot, time will heal it. Blood spots are essentially bruises, and just like a bruise on your leg it will disappear over the healing process. Hope I helped.
