would like to repair blanket with fabric hot glue, but.....

Glue should not outgas because they are just thermoplastics. I did note that some are acrylic copolymers and some acrylic monomers can be smelled below one ppm. It is a level where they are harmless but some may object to smelling anything.

The most common, ethylene/vinyl acetate, hot melts should have zero odor.

I use Bish's Tear Mender for repairs like that. It's a latex product and has a slight odor until it dries but I've never noticed any odor afterwards.

Head down to the local thrift store and pick up a new blankie that isn't worn out.(or learnto sew and mend it properly - cotton thread doesn't outgas and is as "hypoalergeneic" as the rest of the blanket,)

I don't think I'd stick with this. As at least one other said, either sew it or pick up another. I too have experienced the phenomenon you describe. I've glued things in the car, for example, in the past and could still smell the glue for up to a month later. My guess is that summer temps in the car didn't melt the glue, but might have caused the "outgassing" you describe.

A couple months and then try again. If it's still noticeable, the rest of the winter and try next fall.

I'm not sure about this exact case but I normally use contact cement for gluing fabric because it bends.