Microsoft, believe it or not, isn't only supporting Nokia, and the Black firmware update is a NOKIA firmware. The preview of Update 3 is NOT to hold developers and enthusiast until the Black Update. That's just absurd to think about. It is exactly what I said, a refined version of the Beta for Update 3. Microsoft stated that it was stable enough for enthusiast developers and it's the closest direct OTA updates we can get at the moment. If you had any other phones that received Update 3 officially, you can see there is no notable difference in OS Preview (Firmware varies OEM to OEM) from the official launch, it was minor bug fixes when the official release came around.
If the Preview Beta was a BETA preview given to Bloggers, Reviewers and Companies, then everyone who signed up will be running the 8.1 Update right now, but its not. It's for the small guys like you and me, and Microsoft actually recommends you install the preview, and you will receive the official release and firmware from your OEM when the updates are actually available.
If you're looking for the Firmware, then the Preview isn't for you. Its for the people who want Update 3. The firmware for cell phones usually comes baked with the OS Update, but that's not necessarily required. I'm also talking from experience, I have the Update 3 Preview on my Lumia 925 and the official release on my 1020, and there is NO difference in the Operation System, but there is an obvious difference in Nokia's offerings.