My eldest used to HATE the blender. Their hearing can be pretty sensitive sometimes, so maybe that's it. Can you get him to watch tv or something while you use it? Something else I do now is explain that I'm going to turn the appliance on and it's going to make a noise like this (and I do a rubbish job at making whatever noise it is), then I smile and count down and the noise is great fun.

With the blender, I'd do it a bit at a time, like pulse it so they know it's going to stop and the noise isn't so full on. It's a new thing for bub, so you might just need to do a bit at a time to get him used to it? It might create some high pitches he can hear particularly well though, so it just might be annoying at close range for him.

Otherwise, do you have earmuffs bub could wear? Can you do the blending all at once when you're partner's home and him and bub can wander out into the backyard or go for a walk while you blend?