He didn't absorbed , Obito gave him other half .When did Naruto absorb the other Kurama for it to be dark? Well enough of it to be dark.
Naruto is just using the image of Kurama for his avatar, the avatar is not kurama itself
Also Naruto is using the light half of the sages chakra witch is more powerful than Kurama's light half.Naruto is just using the image of Kurama for his avatar, the avatar is not kurama itself