Blackjack is awesome by default
(or well actually masochist by default)When Morning Glory was described when Blackjack found her I had no trouble imagining her with a buzzcut mane (kind of like an even shorter-maned Scootalo. )
Later however it is described how her mane covers her eyes so I figured it grew a bit but still quite a bit shorter than your average pony mane. At least that's how most artists visualize it (: