Firstly Japanese diesel engines have never been the best engines ever built. They're quite sluggish and heavy. Black smoke indicates either too much fuel or too little air. Check the inside of the inlet manifold to see if it's clogged or restricted. Also check the turbo for any signs of wear on the vanes. Don't rule out a blocked exhaust either.

If someone's fitted a K&N air filter, I'd be wondering what else has been done to it. Has it been 'chipped' for example? Do you know its history?

I know this car sjnce 2006, No its not chipped and not much done with this car .... it was owned by a close relative and was kept with a lot of care .... 140k is the total milage. Turbo was changed few months back.