I am somewhat infamous on reddit and sometimes here with my obsession over old Swain, Black Rose, and LeBlanc lore.
But considering Swain's upcoming VGU and everything, I'm hesitant to restate it all again, considering there would be new lore by next year, and honestly I would just wait on that and no longer fully speculate on the old lore between Swain and LeBlanc.
Fair to say, it was very extensive and detailed, the most detailed of any inter-character relationships, but it was obscured by being hidden in the Journal of Justice mostly.
Although, one aspect that's likely to remain 'canon' is the bit shown on the Sion Rebirth Teaser website. LeBlanc alludes to the whole thing with Kalamanda (the Dominion map/event), where she disguised herself to look like Jarvan IV to instigate a battle between Demacia and Noxus and she and Swain pretended to fight (the Journal of Justice had a special big piece of art of the battle, and her face was shown in the reflection on Swain's breastplate).
From their conversation, apparently she, and by extension the Black Rose, has been helping Swain to reach this point. She remarks that Swain doesn't seem to be 'visiting' as much as before, which might hint that Swain is deliberately distancing himself from the Black Rose more now that he's Grand General. Regardless, he asks for her help with waking up Sion, she acquiesces and gets Vladimir to help with that.
Should also be noted, in the old lore Swain is shown to have been part of the Black Rose once. From LeBlanc's original League Judgement, she remembered a scene where she met Swain to discuss, well actually about **him **oddly. Swain wore the Black Rose ring to his meeting with her, congratulated her on becoming Matron, made small talk.
So the implication is that Swain was a member of the Black Rose before, he knew the current LeBlanc before she became Matron, and he met her after she became Matron to discuss further plans. LeBlanc notably held his hand and sounded upset or sad that Swain did something to abandon his previous identity, becoming who/what he is now, referring to his current status + the whole demon raven thing. Swain mentions he made the sacrifices 'for us'. This meeting is really the point the two of them started the plan to get Swain to become the Grand General.
But really, how much of this shared history/connection is going to remain with Swain's VGU? Who knows really. Even though the Sion Reborn teaser seems to confirm at least some aspects do remain, it definitely can shift come next year.
Honestly, LeBlanc would also need some revamping and updates as well to her lore, because at this point it's going to come mostly from Swain's VGU and I don't know how much they can possibly cover.
Personally, I like their relationship. It is absolute fascinating the different ways the old Riot writers insinuated little bits of their actions and relationship in bits of lore, that was just lost on a lot of people because it was a little too obscure.
Like, I'll give another example, once in the Journal of Justice, there was a clearly fake 'reader's letter'. This 'reader's letter' was addressed to LeBlanc, and basically says (paraphrasing a bit) "ravens do not make good garden protectors, they would sooner (leave) once the winds bear them high". This letter was from a disgruntled Black Rose member who clearly a) did not like Swain being involved and b) believed Swain would either abandon or betray them as soon as the opportunity came, and tellingly c) implies that the whole decision to even involve Swain was LeBlanc's decision alone, without full support from all the Black Rose.
So many little things get implied and hinted. I loved that way of lore building. But it made it so spread out, that only those actively seeking these bits out ever realized they were there in the Journal of Justice.
Heck LeBlanc was Swain's date at a ball in his honor, stated by the Journal as really 'hanging off his arm' the whole time . Granted, she may have been an illusion but it's amusing because seriously, how many other champions have legitimately been stated to actually go out together to do anything other then battling?
I welcome more explicit lore, so that I don't have to hunt down obscure little articles like this and have to summarize it for everyone to piece together.
But you kinda lose a lot ambiguity and a kind of depth when you make it more obvious. I always think that 
and 
is one of the best written relationships and interaction simply because we're not entirely sure what exactly is the relationship between them.
