Spot on the dorsal is not an indicator of sex.
Bleheri are very difficult to tell apart, especially at a young age.
The female tends to grow smaller, and have a more rounded head.
The male bigger and a more wedged shape. This is my experience from my pair that have bred.
Also there is only one typ of Channa Bleheri. (or rainbow snakehead)
Names like "Bleheri Blue" are usually trade names for a completely different Snakehead. Usually a Channa sp. Assam. Assams have blue lines in the cadual fin, Bleheri have orange spots.
Post a picture and I can help you with the ID of your snakehead.
Here is one of my Bleheri: