Pretty much as already stated above I am a new player and have spent quite a bit of money on SC's. I only buy one lot weekly to limit myself and when I use to see a black unit I wanted in the spot light I would use all my SC on trying to get them.Only one I have ever managed out of the 2 I chased was Summoner Farne, my other two blacks I got where not the spotlight units. One I got was Sybilla (Who I was very happy to get) but was actually trying to get spotlight units Sophie. The other I got was Berna who also was not a spotlight units.

The amount of times I have come up short handed chasing blacks is numerous, it is all luck based so if you don't mind taking your chances go for it but your not guaranteed a black units and second your not guaranteed the spotlight black unit.

Now onto the princess question, I only have 3 princesses one being Sybilla who is by far my favourite and most used princess. My second is Themis and third is Lillia.

Sybilla is powerful, she has a chance to instant kill enemies and is very strong, being a black you can use her skill quickly so she makes for a powerful princess in my opinion and I look forward to awakening her to get her ranged attack.

Themis & Lilla are in my opinion good princesses for tanking in a one on one fight with their skill, haven't had to use them yet where Sybilla couldn't but I see them as more a tank princess than the others, a good lighting rod if you wanted them to be. Sure Themis after AW because a hard hitter and Lilla will become a better tank with her heal ability.

Olivie is the princess I want the second most after Sybilla, though I see her only to be greatly used if you plan on having an army of elves, might be greatly paired with Spica.

Sherry I am not to sure how she is but she seems to be a strong attack princess but not sure what cases you would need her over Black princesses like Olivie or Sybilla.

So personally I think Sybilla is the best princess but be careful in your pursuit for her as it will be costly if your unlucky. Plus you don't want to feed the devs to much money for lack of content/events