Make sure your air filter is tip top. That can cause black smoke.
The test is for how black the smoke is.
I'll check that and give it a clean (It's a K&N)
The MoT tester shouldn't be hitting your vehicle with anything.... finger and thumb to check for corrosion or at the best the ****ty corrosion assessment tool. Even then it should be used lightly. If you think he has started to prod holes and bash it in with a hammer etc he could be liable for damages.
That is DEFINITELY what I suspect, unless he's got very hard fingers, but there's no way of proving anything
I put money on it your bushes were a pass too.
Your not the only one, I needed a fair old pry bar to get any movement from them