Delightful implementation of the flavour.

However unfortunately I fear just as a mono-black sorcery for 6 mana to make 5 1/1 flying tokens is far too good.

trying to make it so there are clear and obvious advantages to either side, so that a player would actually have a reason to flip it... Im having trouble balancing Sally and the Bats

4 flying power versus 3 double-strike power is a fair and interesting tradeoff. Black can get 4 flying power for 6 mana, so I suggest the front should cost 6 (probably ). A 3-power double striker is clearly strong, but I think 6 then 2 spread across 3 colours is completely fine.

The 'de-transform' seems a bit low, though. By the time you have mana to cast Delphine, you're pretty close to transforming this card three times per round. I don't think I'd ever send Sally into combat, if I could do that.

Remember that every time Sally flips, you exile all the bats, so you're not gaining bats

How is the rarity? Really trying to focus on having well balanced commons and uncommons

Whuh, I was assuming "common" was a placeholder rarity here. There's no way a 3/4 double striker should be common. That's the opposite of well balanced.

But yeah, there's no benefit to transforming her multiple times per round. (Though there's a scary trick with the instant-speed transform - attack with her as a flyer, then once she's unblocked, transform her for 6 to the face.)

Oh, whoops. Guess there's always sacrifice outlets, but yes, not the problem I thought it was.

Yeah, this fails the common test on splashyness, words, board complexity, efficiency, power level and being a single named individual.

This, moi-san, is a mythic.

Nice and flavourful, though.

Thanks for the advice; any issue having one side be legendary and the other not?

No rules problem. Some slight weirdnesses if there are two of them on the battlefield at once, but nothing too bad now that the legend rule has changed to only look at each player.