Thanks really cool I did not know that and thanks for the link. I'll have to keep an eye on his arms now to figure out.I suspect the "dark spots" you are noticing are the gill hearts and won't help with id. Most cephalopods, octopus, cuttlefish and squid (but not nautilus) have 3 hearts, a centeral (systemic) heart that moves the blood around the body and two gill (brachial) hearts that pump oxygenated blood to the central heart.
(Image from http://nerdy-girls.org/2014/06/27/things-i-learned-from-cephalopod-week/)
For sexing your new ward, have a look at this post.