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Its true that just having dark coloring doesnt have to mean dark type. But think about this:
If you had never seen luxray and didn't know its typing what would you guess it was?
I wouldn't think pure electric.
Now do the same with crawdaunt. I know I wouldn't guess it was part dark.
Luxray doesn't have to be dark electric but it would totally fit the design, the dex entry and its moves.