The darkening of the growth point, as shown by the photographs shared here, is caused by humic acids, and perhaps other solubles, wicking up and depositing themselves, first on the hairs and stipules of the leaf primordia, eventually covering the entire surface of the growth point and leaf primordia. My experience is that this can have a damaging effect on the growing point and can supress new growth. The easiest way to reduce this is to gently provide overhead watering with warm, purified water as often as necessary to reduce this precipitate. Another solution would be to use less peat moss, since it is a strong source for the most offending compounds, though other media ingredients may also be sources. It is also affected by temperature, ambient humidity, air movement, etc.
