That's pretty much how it went.
Keep in mind Harry Potter Fanfiction tried everything. To put it in perspective the oldest 100,000 word HP fics are dated back to the year 2000. Blood Prince came out in 2009. Hell, there are a bunch of year six fics written before that book came out.
Point is, as shipping goes, Harry got paired with everyone. Everyone. Harry/Slytherin girl becomes a thing because, well, bad girls are hot and all that, and Harry in Slytherin plot lines have been a thing ever since book 2. Problem was, out of the Slytherin girls we had actually seen Millicent was noted as troll-like (not that it stopped some fics), and Pansy was pug-nosed on top of being Draco's maybe, sorta girlfriend. The other girls were basically blanks slates though and thus fair game.
Blaize was lacking in any sort of physical description and had a cool unisex name. Somehow she was the one that won out in the fandom and it became a trend. There were male! Blaize fics as well, but by the time he was revealed as he, there were a ton of fem! Blaize fics. It was almost a given in the fanfiction community of the time.