I believe you misunderstand.
Firmware updates are named alphabetically and typically correspond with a Windows Phone update.
Windows Phone 8.0 GDR2 comes with Lumia Amber
Windows Phone 8.0 Update 3/GDR3 comes with Lumia Black
Windows Phone 8.1 comes with Lumia Cyan.
Now with the Developer Preview program, you can install new updates outside of and before the normal public release. This does not include a firmware as its provided by Microsoft. Manufacturer's deal with firmware.
So this means that having Windows Phone 8.1 with Lumia Black firmware is possible. But what you need to know is that Black is a downgrade, not an upgrade as you seem to believe.
The 630 comes with 8.1 + Cyan. There is no 8.0 for this phone, and asking for Black is to be asking for a downgrade and something that does not exist for that model.
What you should be asking is when the 925 will receive Cyan, not whether the 630 will receive Black.