Outer Limits 4x4 Board

Just curious cause it doesnt make sense to me, but can someone explain to me why a diesel blows smoke when the fuel filter is blocked?
In my way of thinking it would be getting less fuel so why does it smoke black??
I have a td42 running 12 psi

The timing advance on the pump works by fuel pressure. So if there is not enough fuel to work the advance it will smoke at higher revs.