Sorry but regular emmissions of black smoke is a problem - it is a sign of incomplete combustion, so it indicates that something is amiss in your engine at that point. That may not sound like a problem short-term (except for the car behind!), but long-term it certainly is and it will damage your engine, no question. Just because lots of diesels produce smoke does not mean that that therefore is acceptable or nothing to worry about.
Lots of petrol cars 'pink'; people do nothing about it because they think "oh lots of cars pink", but the fact is, it's slowy but surely killing the engine. If a petrol engine pinks regularly, you should investigate. If a diesel engine smokes regularly, you should investigate. Just because the MOT emissions test only looks at particulates on no-load doesn't mean there's not something going on with your engine!
It sounds like that as Stealth are doing custom remaps, then you could ask them to ensure they don't push it chasing big BHP, that way you'll minimise smoke.