Smoking the light cluster is a 'grey area' as the MoT tester must use his judgement as to whether the light output is significantly reduced.
The rear reflectors are slightly different as they must reflect a red light to comply with the regs, so I can't see how blacked or heavily smoked can pass
"You must inspect the 2 mandatory red rear reflectors that must be fitted."
https://www.gov.uk/guidance/mot-inspection-manual-for-private-passenger-and-light-commercial-vehicles/4-lamps-reflectors-and-electrical-equipment#section-4-3-1