This started as a cutting, and recently I have seen some blackened, dried out sections, especially in the indentations on the stem. I am wondering how I should proceed with this. (I am assuming it is an epiphillum , because it looks like my other ones.) Just FYI, it is in a south window. There are also two epiphyllum oxypetalums in that window, which bloom constantly so I thought this one would be fine there. I only have south light, and I do not have outside access. It is potted in cactus soil with some extra perlite added.
This does appear to be an Epiphyllum, but this will be hard to say for sure. It is either that or one of many closely related species. Differentiation is difficult. Many cannot handle full sun during the peak of the day. This could be a case of sunburn. You can try by letting it get some shade around the middle of the day. If it continues, then you may want to use a fungicide. (I would, just in case, any way)
This collection of articles will help you with the care of your cacti: