XP Oct Moms: What do you know about dark circles?

e_delo

LO has dark circles around his eyes. They come and go but when they're there, the circles are so dark it looks like he has black eyes. I've googled it and only found that it usually means allergies but it could be anemia. I am concerned because my step niece (not blood related) had a hole in her heart and the way we are able to tell that she isn't feeling well is by the dark circles around her eyes. But she is older (12) but the hole was found accidently when she was 10. Does anyone else's LOs have dark circles around his/her eyes? Or has anyone heard or have experience with this?