That is mostly fuel smoke, and in this case it means nothing.
A basic mechanically injected diesel injects fuel primarily based on two things: RPM, and 'throttle' aka how far you pull the lever.
It does not compensate in any way for the engine's ability to burn the fuel. I.e., just like a carburetor it would be a miracle if the amount of fuel that came out of it just happened to be the exact amount of fuel the engine could actually combust at every point through the rpm and load range. In reality it's NOT the exact amount. Usually it's more, because it's safer on a gas engine to run rich than lean. Diesels operate a bit differently in that lean is pretty harmless (just leaves power on the table) but too rich can be bad. When not under load a diesel actually settles at a given rpm by burning all the fuel you give it. Ie, if you want 2000 rpm you give it the amount of fuel it takes to overcome the friction of spinning everything at 2000rpm, and it spins to that rpm. At that point it's extremely lean, just 'breaking even' and burning only enough fuel to spin itself. As you add load, the engine slows down and the 'governor' mechanism adds fuel to compensate for the load and tries to keep the engine at that rpm.
Mechanical injection pumps also are generally set up to err on the side of 'too much'. Since they have no feedback based on engine load, exhaust temperature, amount of oxygen in the exhaust etc etc etc, most of the time, just pulling the lever all the way initially results in 'too much' fuel. When you pull the lever rapidly at low rpm, you inject more fuel than the engine has air to burn it with, and you get visible smoke out the exhaust. As the engine accelerates, the difference between the amount of air and the amount of fuel shrinks, and the visible exhaust lightens until it becomes basically clear.
If you repeated the 'roll on' with a slower movement of the lever, the exhaust would never get dark. If you wham the lever back rapidly from low rpm, you'll get a big puff of dark smoke.
It's just a primitive fuel system. But that dark smoke that clears up as the engine accelerates, isn't really hurting anything other than 'the environment' and contaminating the engine oil slightly more quickly. Nothing you'd ever notice. The problem is when it STAYS dark even at max rpm. At that point you could potentially have too high of 'endgas temps' and be overheating things in the cylinder, and 'washing down' your cylinder walls and contaminating the oil pretty rapidly. If it's only smoking when under acceleration or heavy load, but clears up as the engine 'catches up', or any time you're sitting with no load at a fixed rpm, then it's fine!