Thank you all for your responses.
I infer from your remarks it's unlikely a sky blue cock would produce cobalt rather than sky blue babies.
I also want to point out all 10 babies (9 cobalts and 1 sky blue) are YF.
Attached is a picture of the only other candidate father; he's a light green normal (possibly split for opaline). (Could he produce cobalt babies? Could he produce YF babies?) He is bonded with the sky blue dominant pied spangle hen who is the topic of this post and is pictured with the light green.
I highly doubt the light green normal is the father of either hen's babies because:
1. Sky blue cock is bonded with cobalt dilute hen
(see first picture).
a. I observed them mating several times before and during hen's first clutch. I never observed light green mating or trying to mate with hen.
b. Hen's first clutch produced 4 cobalts.
c. I observed them mating several times before and during hen's first clutch. I never observed light green mating or trying to mate with hen.
d. Hen's second clutch produced 4 cobalts.
2. Light green cock is bonded with sky blue hen, but he's obese
(see second picture).
a. Light green cock is obese (53g) and thus may be infertile.
b. Prior to hen's first clutch, I observed light green trying to mate with her only a few times; but he was too fat and fell off her; thereafter, he preferred to "mate" regularly with a surrogate object.
c. Hen's first clutch was 100% infertile.
d. Hen's second clutch was 100% infertile.
e. Hen's third clutch produced 4 fertile eggs -- 1 cobalt and 1 sky blue and 2 DIS.
f. After hen's third clutch, I observed sky blue cock mate with hen. Thus, it's possible he got to her prior to or during her third clutch, which was unexpectedly fertile.
In short, I can offer a lot of circumstantial evidence proving the sky blue cock is father to both hen's babies.