That plant looks very healthy. I couldn't tell you what the brown stem is related to but how often do you water it? Also what kind of solid did you use?

The dark color at the stem joints is normal.

As Jean says, the dark color is normal.

But the overall plant looks pale - not as healthy as it could be. I would recommend that you water less, and if the plant is wilting then move it into some shade during the hottest part of the day.

It is still a young plant and has significant growing to do. As you reduce watering, the plant should be able to take up more nutrients from the mix. The plant is in need of nutrients - whether due to the impaired roots (from excess moisture in the root-zone), or due to an actual deficiency in the mix.

Josh