The brake fluid reservoir on a 1991 Chevy S10 Blazer is typically located
under the hood, near the firewall. It's often a small, cylindrical container with a cap that says "Brake Fluid" or has a brake system symbol on it. Look closely around the master cylinder (the main brake cylinder) – the reservoir is usually attached directly to it or very near.
Important Note: Brake fluid is corrosive and can damage paint. If you spill any, clean it up immediately. Also, always use the correct type of brake fluid specified in your owner's manual. Using the wrong type can severely damage your brake system.