1999 Blazer help!!!
 

Our blazer will not start. The shop got it started the other day, but it hasn't started since. We are in the process of resetting the passcode but after 10 min the security light is still flashing. What next?? Thanks.

https://blazerforum.com/forum/showthread.php?t=41012
That should help

The security light will not shut off at all. We have let it sit on step 2 on the passlock resetting for over 2 hours and the security light still flashes!

The security light will not shut off during the passlock reset. We have left it on step 2 for at least 2 hours, but it won't stop flashing!

https://blazerforum.com/forum/2nd-generation-s-series-1995-2005-tech-41/flashing-security-light-no-start-override-50018/
This may help, or you can just replace the ignition switch ( not the cylinder lock)