I'm actually trying to build my own ROM for the Blu Studio 5.0S (my family all have the same phone now, and I previously had us on Concur One, but there's so much bloatware in there that isn't neccessary it's absurd -- these things have 1GB of RAM, and yet we have less than 1/3 of that to work with once the phone has been running for a few hours, not to mention that they dressed it up as 4.3.1 when it isn't..).
I've actually tried assembling 4.3.1_r1 manually from AOSP and loading the drivers in -- it causes a bootloop. As far as I can tell, nothing beyond 4.1.2 works on these devices thusfar (though I'd love to be proven wrong); all the ROMs that look like they're 4.2 or 4.3 aren't; they just edited the build.prop and other values so it reported being 4.3.1, and looked like it, but wasn't actually. Bit annoying actually.
I've not tried KitKat yet, as I can't seem to build it in any form, and I don't much like the idea of porting another ROM; I'd honestly rather maintain my own for my family based off the Karbonn S5 Stock ROM.
If Karbonn/BLU does come out with a KitKat stock though, I'd love to know that, and I'll likely be watching for it & will share it if I come across something working, would be awesome to build off that instead for my ROM.