A Blind SQL Injection is a type of SQL Injection attack that asks the database true or false questions and determines the answer based on the applications response. This attack is often used when the web application is configured to show generic error messages, but has not mitigated the code that is vulnerable to SQL injection.
It can be used to insert an SQL query into the database to either extract data, modify data or delete data. It is often used to insert unwanted or unauthorized affiliate, spam links, or malware/adware on sites.
If you are on WordPress, there is a good chance you are using this Yoast plugin. To fix the issue, upgrade to version 1.7.4 immediately. This version is documented to be a security fix based on what Ryan Dewhurst found during a security scan. The security fix says:
Security fix: fixed possible CSRF and blind SQL injection vulnerabilities in bulk editor. Added strict sanitation to order_by and order params. Added extra nonce checks on requests sending additional parameters. Minimal capability needed to access the bulk editor is now Editor. Thanks Ryan Dewhurst from WPScan for discovering and responsibly disclosing this issue.
You can learn more about the vulnerability at TheHackerNews.com.
Postscript: Yoast announced that the WordPress team actually automatically pushed an update to WordPress installs that run an older version of this plugin. So many sites running this should be automatically updated.