Yes, but when Blair opening tells Serena in 4x20 that she hadn't been happy for a very long time and that she was sick of Chuck pulling her into darkness, to me, Serena would have taken that as a major sign that Chuck didn't make Blair happy. Whether what Blair told Serena was true or not I don't think really matters, but when your friend tells you that a guy was making her miserable instead of happy you don't continue to try and push them together. I don't think it's really relevant whether Serena likes Chuck or not, but when Blair said that she wasn't happy with him Serena should have stopped pushing Blair in Chuck's direction straight away. That's why I think it was completely OOC for Serena. She's not a bad friend (intentionally) to Blair at all, however IMO the writers are making her look like one in order to push Chair.
I fully believe that Blair wasn't happy with Chuck at the end of S4, but I think Serena's comment in 5.08 was more about acknowledging that Chuck is different from his actions in 4.20 and looking at Blair's relationship with Chuck as a whole. I think Serena realized that Blair was unhappy with Louis after sending out the Gossip Girl e-mails and probably perceived Blair's apprehension to marry him. IMO, it's not like Serena is shoving Chuck down Blair's throat. She's simply saying, "Wow, he's changed and could be everything you want in a man now." And Blair basically reiterated this herself in 5.10 with, "You're all I ever wanted." Also, in 5.10, we see Serena helping Louis find Blair, so it's not like she's doing whatever she can to reunite Chuck and Blair (unlike Dan). I think what changed between 5.08 and 5.10 was Serena's realization of her feelings for Dan.