Makes sense now, I appreciate it. :-D
As Japan I have just taken over China from Kwangtung. Can I now blitz through my new China territory with the stack of unused tanks I have in reserve in Kwangtung and take over Sinkiang or do I have to wait a turn?
You have to wait a turn.
Blitzing is a combat move, where you blitz through an unoccupied territory and take it, either to engage in combat in an adjecent territory or to move (or return) to an adjecent friendly territory.
In your case your combat movement phase has ended, since you have already resolved combat in China. You can not take Sinkiang in this turn then, since it would be a combat move.