Why are butterflies drinking blood?
I didn't watch the video so I don't know if this was mentioned. Butterflies will exhibit a behavior called mud puddling which occurs mainly with male butterflies [1]. Puddling is when butterflies seek out moist surfaces in order to consume the fluid. Butterflies will puddle in soil, dung, carrion (rotting flesh/meat), etc. Boggs and Jackson claim that puddling falls into two categories which aren't mutually exclusive [1]:


scarce nutrients for example sodium
competitive exclusion of males or young individuals from flowers 


But we can see blood isn't the only option for puddling. Basically anything that provides moisture and nutrients will do.