If you have given blood and notice a small plaque on your skin, it may be a normal reaction to skin irritation. You can try applying a fragrance-free moisturizing cream like Aveeno regularly to help with any itching. It's important to avoid scratching the area to prevent slowing down the healing process. Using a fragrance-free soap like Dove and patting your skin dry after showering can also help. If the lesion doesn't improve or grows larger without scratching, it would be best to consult a doctor for further evaluation.