A bird can be visually green but be split for blue body. And if he didn't breed the prior generations he could only tell you specifically what he knew about the current birds he had.
Those split recessives can still be passed on, and would only show up if another bird paired with them had the recessive gene as well.
This is the same reason why you can get visually orange or white breast offspring from birds who are both red, black or purple breast.
You don't have to have yellow to produce blues.
The only mutations that require yellow are obviously yellow, dilutes, pastel blues and silvers.
So the two parents of this baby must both be /Blue body.
The grey isn't a color problem.. that's what baby blues look like!

LOL
Congrats!