I've recently acquired three salvia cuttings that weren't in very good shape and took ages to grow roots, but they finally did so I planted them in some pots.

Two of them are developing a thin layer of white spots on the stem while the other looks like it's bruising black.
They have been in my bedroom receiving indirect sunlight/nice humidity/enough water (maybe too much?)
Just wondering if I'm watering them too much and causing them to rot/mold.

Less humidity, less water.

Salvia likes it moist, but not wet.

Good luck!

I think Gibran's right on this one. My salvia was over watered and looked similar. Good luck! :]

Yeah I don't think Salvia needs that much water. I think when I would over water mine it would get black like that as well. If you give it less water it actually thrives more. Perhaps the roots need more oxygen?? I'm not sure

There's a rule of thumb with Salvia that goes:
Less water more frequently is better than more water less frequently.