Blind SQL Injection is used when a web application is vulnerable to an SQL injection but the results of the injection are not visible to the attacker.
The page with the vulnerability may not be one that displays data but will display differently depending on the results of a logical statement injected into the legitimate SQL statement called for that page.
This type of attack can become time-intensive because a new statement must be crafted for each bit recovered.
There are several tools that can automate these attacks once the location of the vulnerability and the target information has been established