But why does it appear dark?
That's cooler gas from the turbine exhaust used to insulate the engine. The turbine exhaust is running rich (more kerosene, less oxygen) so it will burn cooler as to not melt the turbine blades running at 1500 F.
The gas generator mixture ratio, relative to the engine mixture ratio, is fuel-rich. This provides a lower combustion tempertaure in the uncooled gas generator and in the turbine.
As a result of running rich and cooler it contains a lot of unburned fuel. Soot.
Before it's used to cool the engine, the turbine gases pass through a heat exchanger, shown below. This uses the relatively cool turbine exhaust to heat up oxygen and helium to pressurize the tanks. This further cools the turbine exhaust.
The interior of the nozzle extension is protected from the engine exhaust gas environment (5800 Fahrenheit) by film cooling, using the turbine exhaust gases (1200 Fahrenheit) as the coolant. The gases enter the extension between a continuous outer wall and a shingled inner wall, pass out through injection slots between the shingles, and flow over the surfaces of the shingles forming a boundary layer between the inner wall of the nozzle extension and the hotter exhaust gases existing from the main engine combustion chamber.