I worked with a chemist 15 years ago to develop such a product. A professor had found a salt, Fast Blue B, would change color specific to THC.
We were charged with trying to commercialize this, BUT, we couldn't prove that blood ratio had anything to do with breath concentration.
Breathalyzers for Alcohol are calibrated with an inferred ratio of 2100:1, of blood/breath concentration ratio. This is usually a fairly accurate assumption. The alcohol molecule is very volatile. THC on the other hand is a very different beast. If someone has smoked Marijuana, what you are reading is the residue on the lining of the airways which has a very poor correlation to what is in their blood.
This alone was enough to kill the idea, because ingesting vs smoking would give wildly different results.