If I'm not mistaken, the kanji 黒渦 (Black Eddy) means to Japanese speakers what a Malestrom means to English speakers: A bigass whirlpool (or "Free Vortex") so deep you can't see the bottom.

However, in English, Vortex often carries a cultural connotation of space and/or supernatural dimensions. I think it was a stylistic choice due to the fact that maelstrom still carries strong naval/water roots in free association whereas vortex has been compromised.