For AMCAS:
If you were to list this as Leadership, you'd have to carve out the hours of leadership-y type activities (training and oversite of newbies) and list those dates and hours on their own. How many would that be? Would your official Contact agree? The general community service role would go in a separate space (taking care not to double count the hours).
If you list it only under Volunteering, you could include the dates and hours of everything you've done for the blood bank. You could include both your general role and your leadership role in the name you give the activity space and give details for the latter (date of onset, % time you are in charge) as well as your original role in the narrative. That would make for a meatier activity and demonstrate your increase in responsibility over time.