I think his comment "Well nobody wants to go multiple levels with switching" is based on the general fact that with multiple levels there are always extra physical hops any communication will encounter, so there will be some added delay, plus more hardware/etc. In general, for a VC based campus networks traffic is mostly North-South, which means [generally] very little inter-VC traffic. So most traffic from Access to Core, just hits a Distribution switch, and not another VC switch at the same layer.
As for the RSTP/BPDU Block question, you could implement this on the existing Blue VC downward interfaces (but not the AE to the Distribution layer), such that you now control where and who can add an extra [STP enabled] switch. Yes if you disable/delete RSTP entry for an interface, that interface will not generate BPDUs.
One thing not mentioned (I believe) is that if the new switch is of the same model type as the Blue VC (EX model type not mentioned), you could just as easily extend this switch to be part of the Blue VC. You do this by enabling all the relevant interfaces to run VCPe. Juniper EX allows VC connectivity over Ethernet, as well as by dedicated VC ports. This does not solve the "extra layer or multiple levels situation", but does make management of this [previously] standalone switch, be part of the existing VC, instead of by itself. Maybe a case of 6 of 1, half-a-dozen of the other.
Also, I would not refer your topology as Spine-Leaf. In true Spine-Leaf the Spines are not connected. Your design is just basic long-time used multilayer with each different layer being a VC. Again, 6 of 1, half-a-dozen of the other.
Good luck.