I always took the statement as that in terms of control of a determined amount of Shinsu, nobody will be better than Baek.
So for example - two humans with a pistol each. The most talented/gifted one (Baek) will likely prevail.
In the case of Baek vs Blossom, I think it'd be more akin to a human with a pistol vs a human controlling a tank. Blossom may not have the level of talent/control Baek has, but her power is far stronger, likely nullifying this advantage Baek has.
So I think Blossom wins rather comfortably. Though it's still a very speculative topic without much to go by.