This is a sort of theory I have connecting The Realm of Magic to Eclipsa and Moon.
The Realm of Magic is the source of magic in the universe and the wand is an extention of that. When Toffee corrupted the wand, he in turn corrupted the realm of magic, hence the black goo. The black goo can also be seen in "Divide" when Star wades through it to find her mother. This black goo has to have come from an outside source (such as Toffee or Moon when she was blasted) and this is where Eclipsa plays a larger role.
Eclipsa said that she never learned to use magic without a wand, despite Baby implying that Eclipsa was greatly talented in her magic. Despite this, Moon and Star can turn into their Mewberty forms. The difference between Eclipsa and Moon is that Moon "learned" to become an extention of that Realm through their use of magic while Eclipsa couldn't because there is no universe of Dark Magic. They both have traveled into the Realm of Magic and can manipulate Light Magic (I’ll use the term “Light Magic” to refer to the type of Magic naturally found in the Realm) because they have control of it. The Realm of Magic is full of Light Magic and not intended to be corrupted. Eclipsa, however, meddled heavily in the Dark Arts meaning that she does not have the same connection to Light Magic and The Realm of Magic as Moon and Star. That could explain why she never learned to use magic without a wand: she can't because Dark Magic does not naturally exist the Realm of Magic.
The way I think she can create the Dark Magic in the first place is that she can modify and morph the magic to her will using spells she created through her wand. By extending this logic, this can be used to further to explain the deep purple marks on Moon’s and Eclipsa’s arms. Eclipsa’s interference in Dark Magic would have likely caused this to happen to her as continuous study of it can cause corruption similar to that of the Realm of Magic. Similarly, when Moon made a deal with Eclipsa and used the Death Spell, she meddled with the Dark Arts (of an especially high degree to add) as well, meaning she was also corrupted by Eclipsa’s magic. They both channel a darker side of the Realm of Magic – a realm not present but rather fabricated and advanced by Eclipsa.
Now for something a little crazy… I have a bold prediction for Season 4. I believe that, whether intentional or not, Eclipsa will be like Toffee and corrupt the Realm of Magic. Assuming that the Dark Magic Moon bathed off (I guess) herself remains in the Realm, it can spread and infect it like a virus. Similar to Toffee, he started generally small: he entered the broken wand of Ludo and slowly started to manipulate him while also infecting the Light Magic. This could very well happen with Eclipsa, especially considering that she now owns the wand.
This is probably crazy but I don’t think this is out the realm of possibility. What do y’all think of this?