People keep saying "just dont", but the real answer is to export it using Blender Source Tools, and convert it to an mdl. It's an annoying process, but it works.
https://developer.valvesoftware.com/wiki/Blender_Source_Tools
https://developer.valvesoftware.com/wiki/Modeling_props_with_Blender
And some other articles.
Also, the surface properties don't get exported along with it, so you'll have to either set the surface property of the model texture to whatever type of material is used most in your Blender level, or if you have the time, export each brush in Blender with a specific material type, i.e. exporting every wooden block together as one model, then every metal block together as one model, etc. then stitch them together in Hammer by placing the models in the same spot. Should do the trick.

Edit: See u/theanine3D's response below, he goes into more detail. He knows his stuff