Hi.. The commonest vein that is used during blood collection when donating blood is the median cubital vein.. This vein is quite superficial vein located in the inner aspect of the elbow. There are no major nerves near this vein which can be injured during a needle prick. Since you mention the numbness is only in the fore arm and not in the palm or below the wrist, a neurological damage would have to be higher up in the arm and not near the elbow.. So it is unlikely that the numbness you are experiencing may be due to the blood donation.. You can get a neurological examination done to know the location of injury if any..