Hanging a wooden venetian blind in a square bay
TL;DR: You're over thinking this. The manufacturer has already made the calculations necessary and has determined that the brackets, screws and anchors (if provided, or purchased separately as specified in the instructions) are sufficient for holding up the blinds in any of the mounting situations specified in the instructions. If they hadn't, they'd be getting lots of returns and/or lawsuits (especially in America - we like our lawsuits...).
If you're mounting it outside the bounds of what's specified in the instructions, you're a bit on your own, but still don't have much to worry about as you'll see if you read on...

The manufacturer knows the weight of the blinds you purchased and the brackets for installing it should be sufficient in number (2 or 3 brackets -or more- depending on the width of the blind) and construction (plastic, thin metal, or heavier metal) to support the weight of the blind.
The use of the provided wall anchors (usually I've found they provide them) in the spacing specified in the instructions and the holes specified in the brackets (they usually have different holes for different installation situations) should be sufficient. If they don't provide anchors, pick some up at your local hardware store/home improvement center.
Drywall/plaster board anchors are really pretty darn sturdy and will support a fair amount of weight, but if you're really concerned about the provided anchors, go get some that are rated for higher weights. You will probably find anchors that are each rated to support the full weight of your blinds. When multiples are used (one for each appropriate hole in the multiple brackets necessary to mount the blind) you'll have far more holding power than you need.
Consider that if the larger blind weighs 40 pounds and requires 8 screws (4 in each of 2 brackets as I've commonly found), each screw/anchor combo only has to support five pounds. Sure, there's some extra weight from tugging it up and down, but you'd be hard pressed to find a drywall anchor only rated at a mere 5 pounds.