Hi Cynthia,
I do not have any research to back up what I am saying; just my own observations. William Lippincott did some research with barring on blue but I do not have his paper. The most I can gather from research is that barring is less precise; what ever that means?
It seems as though the blue gene has some control over the expression of the barring gene. There also may be another gene or genes that also modify the barring in combination with the blue gene. The modifier could be the E locus gene; like brown, birchen, or extended black.
I have seen zonal barring (certain parts of the body barred and the rest of the bird is blue) in other blue birds. It may be that the blue gene in combination with other modifiers could turn off the barring gene in blue birds also.
This would account for the blue birds not being barred or zonal barred, then the bird could pass on the barring gene to a bird that was not blue(in your case black).
Remember this is conjecture on my part; I am only trying to give a possible explanation based upon my own observations. This would be my hypothesis.
I have to go now. I noticed there are some PMs in my box. I will answer them later today.
Tim