Question

[Revils]

Can I use  {Crypt Ghast} in my blue black commander? Thanks in advance!

[Dudecore]

Can I use  {Crypt Ghast} in my blue black commander? Thanks in advance!

Crypt Ghast is black. Reminder/Rules text is not considered for color identity in Commander (which is the reason that Wizards designed Extort that way)

[Keyeto]

DC nailed it! And here's the ruling, to back him up: 

903.4. The Commander variant uses color identity to determine what cards can be in a deck with a certain commander. The color identity of a card is the color or colors of any mana symbols in that card's mana cost or rules text, plus any colors defined by its characteristic-defining abilities (see rule 604.3) or color indicator (see rule 204).

903.4b Reminder text is ignored when determining a card's color identity. See rule 207.2.

[Revils]

Ok so I can use it. Thanks very much guys.