I recently became the proud owner of a couple young sundews after spending time learning about their quirks and wonders. Placed an order online and in came a Drosera graomogolensis and a "Drosera x Marston Dragon". I followed tips to help them acclimate to their new environment, including bagging them to help acclimate to humidity and controlling how much light they get after shipping.
They seem to be healthy since they are producing dew and growing new leaves. However, I'm a bit concerned about the D. graomogolensis. While it is now growing new leaves, the back of the leaves turn a rusty brown, which gradually becomes black. The leaves still unfurl and the tentacles look healthy so far, but pretty much the entire back of the leaf becomes black. An interesting note is the perlite on the surface of the media seems to be having the same color change, rusty brown to dark brown/black. (Here are some photos)
My personal thought is it couldn't be sunburn, since admittedly, it probably doesn't get the optimal amount of direct sunlight. It is on a north-facing garden window and only gets direct sun in the evening for a few hours at most. The rest of the day is bright indirect light.
I have rinsed the media and am watering it using the tray method with reverse osmosis water. Daytime temperature is roughly 80 F, nighttime around 70 F. Not sure what the humidity in my home is.. subjectively I would describe it as not humid.
What could be causing the leaves to turn black?