When building my cabinet - I did a lot of research on ventilation. I am by no way a pro but a few things you could try doing to block the light on the back of the cabinet would be to build a light baffle like shown on this post I found
here. You could even build something on the inside of the cabinet so its not showing on the outside. (Vent into your utility cabinet with a baffle built between the flower room and utility area the suck the air out kind of like what I did with mine.
Dark room louvers are great - I got the 8x8 from B&H.