Duanes' fish (pictured above) is definately a female and is a good reference... but do note that the blue/black area on the dorsal fin isn't always as large as in hhis pic. Sometimes up to half the dorsal fin is dark, sometimes it is only a round spot the size of a dime...
Also, immature fish may have not developed this distinguishing mark yet, although Fishbee's (OP's) fish look like they should be mature enough to sex.