Anyone who can write is a potential blogger.
You just need words...
...and a blog
...and an idea that you can write about
But you want to know the DO'd and DONT's for a Fashion Blogger... right?
So you can...
...write what is in-fashion and keep readers happy
...write what isn't in-fashion and keep readers happy
...write about what's good in fashion
...or what's bad in fashion
...or what's ugly in fashion
The point I'm trying to make is that you need to give a COMPLETE picture of fashion and all that connects to fashion. But then, if you're fussy about a word called a 'niche' then you might as well focus only on a small part and go deeper than anyone has ever gone.
The super secret formula is: keep writing.