Starts fine, misfire cold, then fine - ideas?

[bherlingo]

Hello,
A lengthy explanation follows! I have a Berlingo with DW8 1.9 Diesel in it. Starts immediately and no prob, tickover perfect - but if I raise the revs slightly it misfires and black smokes. Back to tickover, perfect. After approx 3 minutes it runs perfectly without a hitch. When I was at the front of it today it was ticking over warming up and I detected a slight change in the engine note, possibly it was not the engine but a change in where the air was being drawn in - I knew it was going to be the transition from this odd running to perfect and it was. Does the engine draw in differently when cold? Its as if it has enough air to run on tickover and deprived if i rev it (hence the black smoke etc) until warmed when another air source opens. Within the air trunking I notice a black box affair in the centre front of the engine to the left of the air filter housing. It tee's off the air intake - what is this please and could it be contributing to this problem?
All help appreciated - ps its a 2004