1: color cordination. The show is big on colors, and since their accent colors corrispond with eachothers color scheme, its pure aesthetics. It just looks incredibly perfect. (Blake: black/white/ purple *splash of yellow eyes so she stands out. and Yang: Yellow/brown/orange *splash of purple so she stands out.) its similar to the color theory in ren and nora; his is a splash of pink, and her eyes being a shade of jade since rens color scheme is jade green. Making them finally "together" together in v4 :3 ❤
RWBY is heavily colored themed. They are color cordinated, unlike blake with sun, so aesthetic wise, they are perfectly paired.
2: personality (with sun as well) opposites attract, so the party girl and the book worm complete the cute sterotype. Whereas sun and blake nearly complete it, theres too much of her getting angry with him for it too look as cute as when yangs light hearted living free way actually HELPS blake. Ex: they talk they had in he empty classroom. With this, picture blake as the moon. There is purposely two suns; the guy NAMED SUN, or the golden girl who burns like one. Both cute, probably done purposely, but it goes down to a matter of which light she leans for. Theres too much of her not reciprocating his feelings and being mean, and not enough sentimental interactions like yang and blake have. Thus why some ship bumblebee over blacksun. Cuz both fit the personalty sterotype, but only 1 portrays it in a more heartwarming and sentimental way than comedic /ex: sun pestering her.
3: due to the similar trauma of V3, they both share a turmoil of feelings no1 else in the show shares yet; INTENSE feelings of guilt, worry, and anger, so since they ARE half the OG show of the team, its a given this feelings will be fixed or get worse, or get worse before they get fixed. BB shippers stand in the hopes of the last or first one; they are already BEST friends (proof by how well Yang knows blake enough to believe she will come to the dance despite everyone giving up on her or losing hope, including sun) So there is a lot of history and feelings between them to be discussed in the future; a lot of set up for either the STRONGEST best friend relationship, or set up for a romance to thrive.
4: trust. yang and Blake share the same level of trust. Ex: When Blake ran away in vol 1, yang's priority was to find Blake, whether she was part of the white fang or not since she trusts her on an emotional level. And When yang was being accused of hurting mercury purposely in v3s tournament, Yeah blake was hesitant to believe yang, but it was within good reason to be cautious for someone who dealt with the trauma and hurt of adams betrayal. She said she WANTED to trust yang, asked once more, then took her word for it imediately because despite the risk, Blake trusts yang on an EMOTIONAL level which is important. Bumblebees trust can be both emotional and physical, Whereas with blacksun's trust bond, its based on the fact that are mutually faunus and he had her back in combat before which shows he could protect her. BBs trust is both physical (since one must trust a teammate) and emotional since they have chosen to trust eachother over scary topics and bad history. Both BS and BB have a good healthy relationship of trust, but emotional and physical trust is important in a relationship.
5: where as BlackSun shippers have the cute moment of her blushing from the attention she recieved when suns team won, and the "my hero" line, bumblebee shippers have a collection of smaller moments that show POTENTIAL for something bigger in the future. Ex: "i love it when youre fiesty" and blake smiling with lightly flushed cheeks to it in v2, or their serious talk with ended with "ill save you a dance" or overall, the communication skills they shared and yang knowing how to get through to her in that tough time. Or blakes jab at yang to keep her in line when they first met penny.
Pretty much, whereas BS shows sun forcing his feelings on her, only to be reciprocated 1/3 of the time, bumblebee just has built up best friend moments that show how well they work together, leaving with a big pile of strong feelings (no1 else shares yet, and doesnt have to be romantic feeling, just a LARGE AMOUNT of feelings left from the trauma and her leaving) that need to be discussed and fixed from v3. Some Bumblebee shippers are...extremely overboard and become the toxic extremists or ship them just because of the negative reasons youre thinking, but some (like me) just sees the blossoming potential that makes them a perfect pair with signs that hint that it could be a thing in the future.
Black sun is cute; i love how their semblences are like yin and yang, however,
Bumblebee has so much potential and ive seen more with them that BS. They were Best friends, and not only acted like a good pair, but even LOOK that way color scheme wise. Shes too mean to sun for me to see much happenin. Even in the intro he glomped her, got a glare, she looked away worried and it just panned to yang. Black suns cute and comedic
But Bumblebee is thriving into quite the love story ❤