That would mean that one cylinder is not fireing. From any number of issues. Could be lack of compression or a bad injector that is dumping too much fuel and not atomizing it proper and just driping it in...out of time somehow???
But the black smoke is either an over fuel issue or fuel that is not combusted. the black is basically incomplete combusted fuel coming out of exhaust.
My theory is your not fireing on a cylinder...thus the reason your throttle is wide open but your motor starts slow and then gains speed yet still blows the smoke. The engine can run on one cylinder...but should have a "miss" sound to it and a puff of smoke on the missing cylinder. But as the engine slowly gains momentum the RPMS would increase but so would the smoke as it sucks more fuel into that cylinder yet it still wont fire on it. But then when you try to load it up you dont have any power cause you lack the torque you need to produce it as your only fireing on one cylinder, and the load drags the engine down.
You can crack the injector lines and see if it cuts off on one cylinder and not the other.If one cylinder produces no noticeable difference when the fuel is coming out of the line when you open it thats the bad one, if one cylinder or both produce a change in engine speed you found there both fireing. If you crack one line and the engine shuts off you likely found your one cylinder that was fireing as you took its fuel supply by opeining the injector line thus taking the fuel from the one cylinder not allowing it to run.
OR you can pull of the muffler and the tiny exhaust manifold and then crank the engine and see which cylinder is puffing the smoke, which will tell you which cylinder is the problem one, but this does not mean that you dont have compression on that cylinder just gives you a place to start.
You probably cant follow my scatterbrained post...as my fingers type way slower than my mind wants it to and the thoughts come to me.