Let's say I have a Sliver Legion on the battlefield with an Executioner's Hood equipped on it, I also have a Cover of Darkness out with it set on Sliver creatures. My opponent has a mono-colored black creature, let's say a Juzam Djinn. Would my opponent's Juzam Djinn satisfy both the fear and intimidate blocking requirements if I attack with my Sliver Legion if he were to declare his Juzam Djinn as a blocker?
If I had a Dormant Sliver out then my opponent would need a black and green creature or a black and blue creature or an artifact creature of any color to block my Dormant Sliver, correct?
In order to block a creature must satisfy all blocking requirements. So if a creature has both intimidate and fear in order to block a creature would have to be either an artifact or both black and to share a color with the attacking creature.
A monoblack creature can block Sliver Legion as you described.
To block a Dormant Sliver you would need a black/blue or black/green creature as you described.
I know it was just for illustration purposes, but Dormant Sliver typically can't attack (due to it giving all your slivers Defender) so your opponent wouldn't have to block anything anyway. Otherwise the answer genini2 gave is correct.
If I had a Dormant Sliver out then my opponent would need a black and green creature or a black and blue creature or an artifact creature of any color to block my Dormant Sliver, correct?
A monoblack creature can block Sliver Legion as you described.
To block a Dormant Sliver you would need a black/blue or black/green creature as you described.