From IFWiki

Blorb is an "IF resource collection format standard", developed by Andrew Plotkin (Zarf). That is, Blorb specifies a method of associating and bundling multimedia resources such as sound, music, and graphics into a story file. The name comes from one of the Infocom spells.

Blorb was originally designed for Z-machine games, but its specifications have since been extended to handle Glulx games, and possibly other game formats as well. Z-machine blorb files typically end with either the .zblorb or .zlb extension; likewise, Glulx blorb files end with .gblorb or .glb. General extension is typically either .blorb or .blb.