I always noticed Sonja was dark as well when she was digesting a big boa meal so I assume that it's a function of drawing more heat to them. I know when we first brought her in as a rescue she was so dark you could not make out her saddles from her background color so I can only take from that either she was kept in a far too cold enclosure or was dark due to the stress of poor care and a burgeoning case of belly rot.

Told you she's an aggressive feeder Becky LOL. That's one big ole female that loves her bunnie dinner (or big rats or guinea pigs or left over oversized feeder rats or...or...)