black tongue on 9 month old baby

my baby is 9 months old. half of her tongue down towards the back has big black spots. i noticed it about 1 month ago, but i thought my husband gave her some blueberries,she was eating quite a few at that time. so this past saturday i saw it again, its not something u can see unless ur looking for it. the front half of the tongue is normal pink, but towards the back is dark. i tried to wipe it with a cloth and some dark stuff came off on the cloth,like dark papillae, but the tongue is still the same,the spot didnt come off, like a stain or something. we went to the doctor today. "he's never seen this" he said. i am worried. im scared that its been like that ever since i saw it the first time. she is a very healthy baby. never had any problems with her. not even a cold. thats why i probably disregarded the darkness the first time. we are europeans and we have no history in our families of something like that. anybody has any idea what can it be? thank you .