This isn't correct. GDR3 is Microsoft's update to the OS. Black is Nokia's update, which can include (but I don't know what all it includes) software, firmware, and changes to the OS that Nokia is permitted by Microsoft to make. Black and GDR3 are two separate updates, created by two separate companies, that happen to be delivered together (for those who have not already installed GDR3 using the developer preview program). For those who have not already installed GDR3, once installed they will find that the OS build number will be the same as the OS build number that those of us already running GDR3 have. Assuming Microsoft doesn't push another update between now and then, the OS build for 928 will be (and already is for some of us)
8.0.10521.155. This is the same build number that many of the updated phones have received through carriers (though MS did put out two different builds, depending on which phone it is on).
Don't be discouraged...
Availability in North America - Nokia