Do mosquitos excrete blood?
When mosquitos feed on blood they release water droplets, this is so they can condense the amount of nutrients from the blood but dispose of the unnecessary water weight. This act of releasing excess water is often mistaken as 'pooping', a clearer view of the process here. Actual footage of fecal excretion however, is harder to come across.

Mosquito defecation is more like iron pellets, to maximize their meals they excrete water during or after feeding (in case they need to make a quick get away). Iron is toxic in high doses, so the mosquito excretes it after absorbing the nutrients in the blood, resulting in dark pellets, containing iron and uric acid.