"Requires Windows 7 or later" and not listing XP as compatible can mean any of the following:
- Not tested on Windows XP (Microsoft doesn't support it, so why should we bother)
- Not supported on Windows XP (it may very well work, but it's too much trouble to provide tech support if it doesn't)
- Doesn't predictably run on all Windows XP computers
- Most video drivers for Windows XP won't work with the game
- Uses features of Windows 7 and isn't likely to run on anything earlier, but we can't be bothered to test
- Uses features of Windows 7 like DirectX 11 and isn't likely to work on XP which only has DirectX 9.0c
- Absolutely won't run on anything earlier than Windows 7 no matter what
- Any combination of the above
Unless "Blues and Bullets" is made by the same developer and uses the same game engine as "Morningstar" you shouldn't expect to have the same luck with it.