White LEDs already are blue LEDs. They add that yellow phosphor coating on top to convert some of the blue light into the green/yellow/orange range. You can get any desired balance of blue:yellow, from barely any blue to way more blue than you'd ever want, just by choosing how much/little phosphor is added (these are the color temp options). It's not necessary to add additional blue LEDs. They're quite expensive for one, and they're redundant when you can control the blue level by selecting the right color temp for your whites.
The reds are quite expensive too, but we're forced to add those because the whites can't produce those longer wavelength reds efficiently by themselves.
To make the spectrum adjustable like you're thinking you'll need to buy two ballasts for every light, do some very complex boards that can reroute the led strings, or design a custom dual output ballast. It can be done obviously, but it won't be cost effective. You'd be better off to have separate veg and flower lamps like we always have. They can use the same ballast.
The reds are quite expensive too, but we're forced to add those because the whites can't produce those longer wavelength reds efficiently by themselves.
To make the spectrum adjustable like you're thinking you'll need to buy two ballasts for every light, do some very complex boards that can reroute the led strings, or design a custom dual output ballast. It can be done obviously, but it won't be cost effective. You'd be better off to have separate veg and flower lamps like we always have. They can use the same ballast.
Switching to "flowering only" chips ups the overall "flowering" wattage and lowers the 'versatility.' 
