Sometimes I see Blacksun shippers make posts about how Blake/Sun should have been canon. I can understand why they want it to be, and why they're disappointed that it's not. I do wish they'd express that without attacking Bumbleby, although I can also understand why they do because they think Blacksun had a chance of becoming canon instead, and if not for Bumbleby... However, it did not. There's evidence of that as far back as Volume 3. We'll get to that later on in this post. Under a cut because this does get a bit long. Bear with me, I'm doing my best to be reasonable and civil.
I firmly believe in the ship and let ship philosophy. Despite Bumbleby being my OTP (one true pairing) that I love above all others, I don't care if someone else ships Blacksun. I just have no interest in seeing anything to do with it, so I take measures to avoid it - like add the ship name to the filtered tags and filtered post content in my settings, block tumblr users if necessary, that sort of thing. It's nothing personal, just curating what I see.
I'm fandom old (although not in RWBY fandom itself because Bumbleby becoming canon was what caught my attention in the first place) and I've shipped both canon and non-canon ships. Canon is not inherently better, it's just a joy to have the validation and to see that ship in the source material.
Do I hate Sun? No, I think he's a good friend to Blake, and I don't mind seeing him in the show, I enjoyed reading the CFVY novel he appears in, and I've even had him with a minor role in at least one of my own fanfics. Am I hopelessly biased in favour of Yang and Bumbleby? Absolutely.
I can admit that Blake and Sun had a mutual crush at Beacon, going by the winks and blushes, and in her own words she technically went with him to the Vytal dance as his date. But one technical date does not a relationship make. The point of Blake and Sun was to show that not all crushes progress into romantic relationships, and that friendship is a great thing to come from it.
So what was that evidence in Volume 3 that Blacksun wasn't going to happen?
Sun was standing right next to Weiss when Yang asked if she'd seen Blake. Well within earshot, and he definitely didn't miss Yang running over to ask because he turns to look after hearing her call. If Blacksun was supposed to be canon, he'd have gone with Yang to find Blake, or gone instead of Yang, and he'd have been the target of these words from Adam:
"I will make it my mission to destroy everything you love. Starting with her." (well, him, not her)
That would have pretty clear implications for future romance. Instead it was Yang, and those implications were for Bumbleby.
Could CRWBY have prevented a shipping war between Blacksun and Bumbleby shippers if they'd kept Sun within view while Yang and Weiss talked? Probably not, but it might have made it more obvious that ship had already sunk when it came to becoming canon in future. Don't get me wrong, it's still a valid ship, it's just not canon.
Sun is also seen looking at unconscious!Yang and Blake clinging to her hand, and I think that's the moment he sees it: Blake loves Yang. He stops the flirty winks, and there are no more blushes from Blake. Like he said to Neptune in Volume 6, helping Blake in Menagerie and Haven was never about romance. She needed a friend, and he wanted to be there for her. She certainly doesn't owe him anything, particularly considering that she didn't ask him to follow her to Menagerie.
TLDR: there is evidence as early as Volume 3 that Blacksun was never going to be canon.
That doesn't really matter as far as shipping Blacksun goes because it doesn't need to be canon to be shipped. In the event Bumbleby wasn't canon I'd still ship it regardless, although I probably wouldn't even know RWBY existed without it. Which isn't something I care to imagine, considering I made some friends and met my girlfriend through our mutual love of RWBY and Bumbleby.