File Name- Blaze-client-mod-fabric-1.21.1.jar File

Use the Fabric launcher profile (not Vanilla). The Blaze Client mod should now be active.

Move Blaze-Client-Mod-Fabric-1.21.1.jar into the mods folder. Do not unzip or rename the file. File name- Blaze-Client-Mod-Fabric-1.21.1.jar

Download the specific file named Blaze-Client-Mod-Fabric-1.21.1.jar . Do not use the 1.21 version on a 1.21.1 client—it will crash. Use the Fabric launcher profile (not Vanilla)