I think it was them trying to either destabilize Boram or to gain his trust. Depending on what opinions the Black Rose held of Boram at the time they would stand to gain something in this event. If the goal was to destabilize Boram by putting him in a shit situation with someone who he was close with, they could push for more power and control in Noxus subtely, while if they actually wanted a better standing with Boram they could've worked with the idea of bringing back Sion, however the means, would be a great step towards that, and by gaining his trust it feeds in to the goal of further control and power for them.
Perhaps they didn't realize just how uncontrollable Sion would be in such a state, and worked under the assumption that Boram would just want Sion back in the picture because they were on good terms. Of course this backfires if the bond between Boram and Sion goes deeper and they were more like comrades, in which case the situation goes sour because they just desecrated the corpse and memory of one of Boram's good friends. It kinda depends on how secretive Boram was about his personal life and relationships I suppose.
I imagine that Sion was the tipping point between Boram and the Black Rose though. I can see Boram, after seeing Sion be turned in to a monster by those mages just finally saying "Sod it. No more Black Rose. I'm putting them in the ground." and tried to wipe them out entirely. It might have then been a contributing reason as to why Swain got the Black Rose's assistance in his becoming the Grand General; LeBlanc held some resentment and was more than happy to off Boram for whatever reason, with the added benefit of then having the new Grand General being an ally of yours being a bonus.
Either way I'm at the very least sure that the attempted resurrection of Sion was a power bid by the Black Rose in some form. They saw Sion as an avenue to get with Boram, be that to bring him down, get in his good graces, or both.
