Why not? To sex by looks those are the only things to look at. Males fin tips get longer too.. But if you have 1 fish, u go by the dorsal.. If u have multiple, then you compare head structure and fins after 4" with keen judgement.
Because many female's black blotch isn't very clear at all, it'll be more of a slight black tinge than an entire blotch, and many male has the same slight black tinge, which makes it hard to determine exactly if it's 'black blotch' enough to be female, or just happened to be unique to the male.
Same with fin tips and head structure. Especially those two actually because it's not enough to just compare different specimens - if you get different ones from different strains or just bloodlines, they can vary in their head structure/fin tips naturally and therefore be confusing. Say one bloodline has naturally longer fins, a different one has naturally shorter fins. You get a male from the shorter fin bloodline and a female from the longer fin bloodline, and comparing them, the female in this case can actually have longer fins. Fin tips was used to be a way to sex Convicts once upon a time, but not anymore. Many females now have long trailing fins, no different from a male, no doubt from people going for them more and hence producing more of them. So it'd only work on siblings, and even then it's not definitive. You get two from the same batch for example, and one has a steeper forehead than the other. The key word here is
steeper. There is no cut off or anything of the sort that anyone can go 'yep, 80 degrees inclination, male' or the likes, and so despite the different in forehead both could be male. Or female. Or a male and a female, it can't be certain.
Hence not 100%. You can sort of tell, or guess towards it, and in some cases there is so much black on the dorsal fin or a large nuchal hump for example, that it's almost definitely male. But otherwise the methods can't be used to tell all Texans.