Bicolor refers to an Australian Shepherd that only has two colors. Red or Black with white, and no tan points. Tan points are what is called Agouti expression. Tan points are recessive to the dominant black gene. Dominant black on the K Locus is what causes bicolor. In this case, 'Dominant Black' is just referring to the absence of tan points. The dog's actual color is dependent on the B and E Locus. Reference the photo below to see all the accepted colors of the breed standard.
KB/KB-The dog has two copies of dominant black, no tan point is expressed and all offspring will not have tan points
KB/Ky- The dog does not express tan point and appears bicolor. It carries one copy of dominant black, and one copy of Agouti. 50% of offspring will not have tan points and 50% will have tan points
Ky/Ky- The dog carries two copies of Agouti and will have tan points. Offspring will have tan points as long as both parents are Ky/Ky. This is a recessive gene and therefore is only achievable by putting dogs together that have two copies of it.