Vent/Range Hoods are not light, so you really need to follow the instructions for installing one - you don't want it to take out a section of wall and come crashing down on your cooking surface. Do it right.
Why can't you install blocking? Are you afraid to open the wall? If you're not doing too large an area, I don't think it will be that expensive.

The DATID weighs 37.92 pounds. Four 1/4" toggle bolts will hold 10 pounds each forever. I wouldn't worry about it.

I'd go with what the manufacturer says.

Everytime you turn on the range hood, it is going to slightly vibrate. And when you run that machine on the higher fan settings, I would imagine it would really jiggle if it's not screwed into something solid.

Those bolts might hold it for a while, but I would not trust them for any length of time. I can just imagine the Sheetrock (assuming that is what you have) being worn away little by little. Hopefully, it will just start to sag at first instead of crashing down suddenly one day.

Why is adding blocking not feasible at this time in your remodel?

Here's a thought if you are just trying to avoid any patchwork to your wall.

Mark the wall exactly where the hood will hang. Carefully cut the hole a little smaller then that area. The wall cut out can be smaller then the blocking too. Just partially drill a couple screws in it to grab like handles. Insert the block at an angle, up, then back down. Hold in place and screw it to the studs. You can screw the little piece of sheetrock back over the gap if you want. Everything gets covered by the hood. Hope that is a idea that might work for you.

Op says that he has a wall without studs. Maybe the wall is brick, block, adobe...?

No way can we advise without knowing what the wall is made of.

I was thinking the same. There are plenty of homes without studs. The IKEA UK installation instructions state that they don't include "fixing devices" because it depends on the type of wall.http://www.ikea.com/gb/en/assembly_instructions/uppdrag-wall-mounted-extractor-hood__AA-725982-2.pdf

We have a similar problem. Our geo dome does not have wood in the walls. Foam inside and concrete layer outside...no upper cabinets either