U can easily spot those "black shops", they normally have hugh neon signboards like "SONY", "NIKON", "CANON", and with very common shop names as "Worldwide Cameras", "Budget Electronics" etc, just like Singapore.
The better shops are Man Shing, Wing Shing, (Mongkok area), Tin Cheung (Tsim Sha Tsui area). If u intend to pay in cash, say upfront so they will give u better price.
The big electronic department store is Broadway, u will see it everywhere, they sell latest cameras and handphones etc, fixed prices.