Anecdotally, I know a few vocalists that sound great in these keys and use them as a starting point
For the major scale, there are 7 notes in the scale and only 5 black keys; you also need to skip ti, the 7th note.
For the minor scale ("C#m"), it's worse; only four of the five black keys are part of that scale.
And I would have thought that something intended to be played only on the black keys would be described as using a pentatonic scale anyway?
A digital piano can transpose things to make it “easier” to play.
Cursory google search says that a sitar is traditionally tuned to something useful for c#
I’m curious if C# is one of those notes that lines up nicely with whatever crappy consumer stereos/subs were capable of reasonable reproducing in the 90s as electronic music was taking off and it stuck around as a tribal knowledge for getting more “oomph” out of your tracks.