The Black Level setting is dependent on the source peripheral, and the corresponding connection. HDMI components generally have the ability to process the Black Level settings at the source, and some RGB signals or some other analog signals may make the Black Level option available. YCbCr signals would be able to benefit, for instance, from a Low Black Level. Those aren't sent from Blu-Ray players.

For the HDMI connection AND the Black Level selection to both work, it would be considered redundant, and that's why it's grayed out.