"By administering imagination and spiritual energy which forms the basis of "YIN" power. He would create shape and form from nothingness.
By administering vitality and the physical energy which forms basis of "YANG" power. He would breathe life into that form."
Naruto already has shown us his ability of YANG power when his cloak mode interacted with yamato's wood totems where the totems proceeded to grow further with branches etc. But what about YIN power, is this the first signs of Naruto doing an advanced YIN technique?
Naruto now is using this "illusion" within his mind to manipulate his chakra cloak to create this very dense black ball of chakra, which by the look of things is the same place within his mind where he fought the fox and used toad sage mode throughout the whole fight. Ones imagination is what allows you to create and change illusions just like in genjutsu(illusion techniques) also needs ones imagination to manipulate the illusion. You might say, in way Naruto is using genjutsu on himself, well not exactly but I just wanted to show you the relationship with genjutsu and the basis of "YIN" power.
Kishi has already used gravity quite well and scientifically correct too, chika tensei is a small black ball of chakra and we saw how strong the gravity was with that technique, basically ripping the ground off the earth. This is worth a look, look at the anime
where Nagato forms the black ball, I kid you not it looks like a rasengan forming.
The rasengan becoming a black hole is actually not that far fetched at all. A technique like the rasengan is actually a dense entity because it's solid, meaning it impacts with other objects physically and repels them. So its fine to talk about the rasengan's mass like kishi has been doing so far in the series. So if Naruto puts in the fox's energy(note fox's chakra has been noted to off the charts heavy by the Minato) into the mix, the rasengan may become heavier and heavier and since the rasengan is all about compressing chakra in smaller area it may begin to create a black hole.
As I said earlier black holes have been theorized to be nature's way of manipulating time and space, so it would be cool yet a fitting way for Naruto to get a time-space technique and follow in his dad's footsteps. If Naruto does pull this off this it would be a good way to use sage mode with his fox mode and build on the fusion Naruto hinted during the meeting with Nagato. Since it would be so heavy sage mode can help Naruto hold and move it around, possibly throw it to avoid being sucked in the hole himself via the strong gravity. I think just like how the wind rasnegan was completed via sage mode I think the same will happen with this too, sage chakra is a stronger chakra and I think its reasonable to say its a heavier chakra also, so this will add to the rasengan's mass and create the black hole and hopefully he will be able to move the black hole away from himself like what Nagato did with Chika tensei where it hovers far into the skies to avoid being caught within the gravity himself. This is all depending on how closely kishi is willing to follow the theories physicists have on black holes, I gotta say this all interesting stuff and I can't wait to see the results of this training ark.