I'm a bit confused about this. I know that in the old day (last month) of analog TV, black setup was at 7.5. Now that we are all digital,should I set the BL to 0? My footage is coming out very low in contrast and I think it has to do with this setting. Thanks.

The setting doesn't affect your footage. It affects the analog monitor outputs.

For digital recording you always want it set to zero.