User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-24339098-20130610235144/@comment-3146930-20130610235946

It is hard to tell. Even when zoomed in, you can't really get a good look. Piper and Hazel both have darker skin, as well as long dark hair. The hair itself looks curly like Hazel's, but it could just be the wind twisting Piper's hair to look curly (they are being attacked by a giant turtle after all). Rick Riordan also hasn't posted anything about the cover on his blog, so it is a mystery.