As an RN and spending lots of time researching wounds, I would never reccomend draining a blister.
First, a blister is a pressure sore. When it is intact, it maintains some protective elements for the underlying involved tissue.
Keep friction at a minimum on the affected area. If it is intact still, you can cover it with a clear tranparent dressing like tegaderm. This dressing will minimize friction and sheer allowing the area to slide decreasing the likelyhood of reinjury.
Stop wearing the offending flip flops.
Take a couple of days off riding, a few days will not affect your fitness level. Blisters will generally reabsorb in a few days.
Good luck!
If I can't go fast, at least I look good.