Perhaps the person who gave you the egg could lend some insights to what other breed it could be or what could be mixed in....
The bottom-most chick down in the above diagram is the 'wild type' which is what the genetics of the Cream Legbar would lead us to expect. Ask the person if there is some chance that the chick could be Easter Egger -- they are very nice flock additions -- entertaining and fun and a good addition to a backyard flock. They can lay eggs of any color -- most often green and blue-- Another blue egg layer is Ameraucana.... there are a couple others now a days that lay blue -- My Pet Chicken has one that is a 'super blue egg layer' -- perhaps this chick is one of those. there is also one called Arkansas Blue -- that I think is a blue egg layer....
I think that perhaps these are Arkansas Blues -- from a post on BYC - but not 100% sure. You may want to investigate some of those breeds and look for a match of the chick downs.

Here are a couple of EEs that I once hatched:
They grew up to look like this:
ETA - happened to find one on ClassRoom in the Coop that looks a bit like the pattern on your chick's down:
This one is similar to Db (Db also lightens phaeomelanin in e+ chick down, making it difficult at times to tell S from s+)

Page is here if you want to dig into it:
http://www.the-coop.org/forums/ubbthreads.php?ubb=showflat&Number=91538
Actually now that I look it isn't similar is it? ;o) -- I'm still going to guess a Columbian restrictor is hard at work on that chick's down. Just a couple of references -- the Db gene referenced is 'black tail' and the phaeomelanin - is the red pigment on a chicken.
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