1. Check your owner's manual: This is the most reliable source of information. It will specify the capacity for your particular Blazer's transmission.
2. Look at the transmission dipstick: If you can locate the dipstick (often marked with "ATF"), the amount needed is generally indicated on the dipstick itself.
3. Consult a repair manual: A repair manual specific to your year and model Blazer will give the correct fluid capacity.
Without knowing the specific transmission type (e.g., 700R4, 4L60E), it's impossible to give a definitive answer. Attempting to guess could lead to overfilling or underfilling, both potentially damaging to your transmission.