If you're going to donate blood...
 

I recommend you have a restricted snack. The restricted foods have more carbs, in general, so they will help replenish some of your blood sugar.

My clinic let me donate, when I was on IP before, and I was OK... though I got the tiniest bit more dizzy than normal, afterward. I found that having part of the restricted food before donating and the rest afterward was the best approach for me.