I personally do not think that Blackfire as a character is evil. I do believe that there is some good in her. Overall, I do think that Blackfire as a character is just very misunderstood, underrated, overlooked, & conflicted. She is very similar to many of my favorite animated characters (most of them female), including Soi Fon from Bleach (Mai Waifu), Charmcaster from the original Ben 10 storyline, Star Sapphire from DC Super Hero Girls, & Sour Grapes from Strawberry Shortcake's Berry Bitty Adventures. Both Teen Titans Go! & DC Super Hero Girls depict her in the way that I think she really is as a person.
Blackfire knows deep down that her sister Starfire is better than her at almost everything, such as being the better hero (most importantly) &, ultimately, the better sister. Also, deep down inside Blackfire's body, mind, & soul, she does truly care about and love Starfire as well. However, Blackfire just does not know how to properly show it or reciprocate all of that love & affection back towards Starfire. I think that Blackfire & Starfire's relationship as sisters has elements of the bonds depicted in Bleach (Yoruichi Shihōin & Soi Fon), Marvel's Guardians of the Galaxy (Gamora & Nebula), & Strawberry Shortcake's Berry Bitty Adventures (Sweet Grapes & Sour Grapes). Finally, all of this is coming from the perspective of an older sibling. Therefore, I think that myself and other older siblings admire Blackfire as a character because we can actually relate to her (at least I can). That is why, as of this writing, Blackfire is currently my favorite DCSHG character (ahead of Star Sapphire, Katana, Poison Ivy, and Starfire) and she is very much deserving of a larger role in the upcoming 2018 Cartoon Network TV Series! :)