Blind XSS occurs when an application reads data from a data source stored in a server that interacts with one or more applications and is affected by our direct interactions.

For example, imagine you have an internal application to manage the inventory of a store. In this application, the employees enter the information of different products that are stored in a database server.

The same database is also used by an online store application, which is exposed to the internet for all the customers and needs to read the product information from the initial database.

If a user injects a malicious string into the internal application, it is not important if the online store has properly implemented input validation controls, as the information in the store is read from the same database, so the store will show the attack.

As you can see, this is essentially a stored XSS, but is commonly referenced as blind XSS.