Purple/black tongues of Canadian sphinx

Hi there. I have a Canadian sphinx that gave birth a month ago. 
The kittens are cute and energetic (already walking, jumping and climbing).
I the few last days I have noticed that all the kittens have purple/black tongues.
I didn't look or payed any attention to that before so I can't tell if the color was different before.
Is it normal for cats from this breed to have dark tongues ?
The kittens are breastfeed only.
Thanks with advance for your answer.

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