I emailed Gardus a couple of weeks ago asking the difference. This was their response.
"The black rods are manufactured using a stiffer poly material. The rod is larger in diameter, and hollow cored. They are stiffer, and easily bend to a 30 degree bend. You can bend them 180 degrees without damaging them, but that resiliency is not part of its intended use. These were designed as the best rod for classic chimney sweeping of an open hearth appliance like a fireplace.
The white rods are a solid extrusion of a specific nylon. This rod is thinner diameter than the black rod. It bends easier, and that is part of the design of the product. It will navigate 90 degree elbows with ease. These rods were designed as the best rod for sweeping sealed/controlled combustion wood burning appliances like a wood stove that often has elbows stronger than 30 degrees.
Both rods have the same button lock, and share integrated connectivity; Meaning, you can connect white rods to black rods, and black rods to white rods.
You are welcome to reach out to me personally if you have any further questions."