Can I use the card stormscape familiar in a commander deck without black in it?
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-MadMage
Yes. The Familiar's color identity is blue, so any commander deck whose commander's color identity includes blue can use it. Color identity only includes the colors in a card's mana cost, all color indicators, colors defined by characteristic defining abilities, and colored mana symbols in the card's rules text (for double faced cards both sides of the card are considered, eg. Archangel Avacyn'c color identity is WR due to the back face, Avacyn, the Purifier, having a red color indicator). Color words, mana symbols in reminder text, and basic land types in rules text don't affect color identity. (Basic land types in the type line affect rules text, so will also affect color identity, eg. an Island's color identity is blue.)
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To be clear, nothing in the rules for the Commander variant (under C.R. 903) restricts the colors of spells you can cast or permanents you can control. For example, in a Commander game, those rules don't keep you from—
casting a spell named Stormscape Familiar (e.g., from someone else's hand), or
gaining control of a permanent named Stormscape Familiar,
just because of your commander's color identity (see also this thread).
Using card tags is mandatory in the Magic Rulings forum. Please check your private messages to learn how to use them. I've added them for this post.
-MadMage
Yes. The Familiar's color identity is blue, so any commander deck whose commander's color identity includes blue can use it. Color identity only includes the colors in a card's mana cost, all color indicators, colors defined by characteristic defining abilities, and colored mana symbols in the card's rules text (for double faced cards both sides of the card are considered, eg. Archangel Avacyn'c color identity is WR due to the back face, Avacyn, the Purifier, having a red color indicator). Color words, mana symbols in reminder text, and basic land types in rules text don't affect color identity. (Basic land types in the type line affect rules text, so will also affect color identity, eg. an Island's color identity is blue.)
In the future, please use a more descriptive title for your thread. You are in the rules forum, so we already know you want a ruling. Mentioning cards, formats, etc. gives us a hint as to what your question will be about, and it will help others with similar questions find your thread and our answers. Just look at how the other threads here are titled.
Former Rules Advisor
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EDIT (Jan. 19): Edited.