This trend really has to be addressed ladies because it is go viral lol. Contouring is the art and technique of receding (pushing back) features and either creating more cheekbone structure or thinning out other features. Basically creating MATTE "shadow" is what we're trying to accomplish. And shadows have greyish/ashy undertones correct?
Bronzing products are to warm the face and have either red, orange, or golden undertones in them. So using a bronzer to contour just creates an unnatural contour looking fake and poorling done. Especially if they are not matte. NOTE: There are SOME bronzers on the market that have grey/neutral undertones and are matte. See my recommended products.
The proper way is to contour with a contouring powder, cream, or liquid with neutral or grey undertones THEN you can go over it slightly with a bronzer to add warmth. Create a "3" on each side of the face when bronzing. Never in the middle. It will make you look muddy very quickly.
My fav CONTOURING powders:
The Balm - Bahama Mama
NYX - Taupe Blush (confusing I know. But its amazing trust me)
Kevin Aucoin - Sculpting Powders "Medium"
MAC - Blunt Blush (medium skin)
Make Up For Ever - Sculpting Kit 2 (light to medium)
Make Up For Ever - Sculpting Kit 3&4 (medium to dark)
Smashbox - Step-by-Step Sculpting Kit
Benefit - Hoola (bronzer technically, but amazing for light skintones. A little warm but it's passable)
You can also use creams or liquid foundations as long as its 2-4 times darker than your skintone and has NEUTRAL undertones.
Bobbi Brown Foundation Stick
Maybelline Fit Me Foundation
Cinema Secrets Foundation
I hope this helps and as always if you should gave any questions feel free to message me :)