A black #2 is a black Doberman that is also capable of producing blue Dobermans if bred to the right (or wrong!) dog.  A black #3 is a black Doberman that is also capable of producing red Dobermans if bred to another Doberman capable of producing reds.

The colour inheritance chart is here
http://dpca.org/breed/breed_color.htm

Two years of age is essentially the minimum age at which a dam should be bred.  Certain health tests like OFA hips and elbows can't be done until then.  The first cardiac exam is recommended at that age as the heart won't be fully mature in size until then.  That said, this is a breed with a lot of longevity problems so I prefer to give dams more time to show me a problem before I breed them.  Although I have bred a couple at 3 years of age, I prefer to wait until they are 4 and are physically and mentally mature and fully health tested.  For me, that means OFA hip and elbow evaluations, vWD DNA test, cardiac ultrasound (by this time, my bitches would have had at least 2 of them, maybe 3), cardiac Holter monitor (again, by this time, my bitches would have probably 3 of them), full thyroid panel, CERF eye exam, liver and kidney panel, CBC and brucellosis test.