I want to have oven roasted tomatoes that have some black on them for salsa. I have over 100 tomatoes and don't want to use the oven for a full day doing this. I think a blowtorch might blacken the tomatoes in a good way. Would this have the desired effect (i.e., having blackened charred tomatoes)? How would it be different from baking them in an oven?
I know the tomatoes won't lose as much liquid as if they were baked, but that is not an issue since I'll be reducing them in a salsa anyway.
Does your oven have a fan-grill?
30-ish tomatoes on a tray, 3 trays, set to fan grill & rotate the trays every few mins.
Alternatively, use the regular oven & when the tomatoes are nearly done, switch to the grill. Rotate as above.
It depends on what effect you're going for.
If all you want is some burnt bits? Sure, go for it. Heck, you don't even have to use the tomatoes for that; you can just burn some paper, or grind up some lump charcoal. (I wouldn't suggest using briquettes or self-lighting charcoal for that.)
If you're going for the taste of roasted tomatoes, that won't be there. That develops through a longer cooking time (though not all day. See Tetsujin's answer) and can't really be short-cutted. A kitchen torch doesn't deliver much heat compared to an oven, just very focused heat.
The other thing you might consider, particularly if your oven is low-powered and/or doesn't have a convection mode, is roasting some of the tomatoes and leaving the others raw. I've actually made salsa like this before, because I like the taste of roasted tomatoes and also the tang of fresh ones. It lets you absolutely roast the hell out of the ones you're roasting.
