Is pledging to donate blood upon success considered a "vow" (nadhr)? And if I am prevented from donating due to a hemoglobin level of 11, what should I do?

The words you uttered do not constitute a vow (nadhr), but rather a mere promise. A vow is only binding when expressed in words that indicate commitment. Donating blood is not an act for which a vow is required. Therefore, you do not owe any expiation (kaffarah), nor are you obligated to do anything.