Blackouts and fainting are caused by Orthostatic Hypotension: a lowering of Blood pressure while fasting. The standard advice is arise from a sitting or lying down position very slowly to counteract these possibilities.
Blackouts and fainting can cause a serious injury unless you are more careful.
Fasting (physiological rest) is also known as the "rest cure" which is required when fasting: rest, rest and yet more rest.