Prevention is best, so ensure your running shoes have the best fit, increase your mileage gradually, and ensure adequate cushioning. If you do develop a black toenail, take a break from your running routine and wear open-toe shoes during the healing process to prevent further damage. Beyond that, the only thing you can do is wait. If it is excessively painful or is not improving, see a healthcare professional. They may be able to drain the blood collection which can provide relief.