I was wondering: when it was Beth McIntyre the swan queen, did she play the white swan, the black swan, or both ?

Given that the poster outside the theatre showed her as the white swan, and Thomas says that they have never had a ballerina play both the white and the black swan before, I think it is safe to assume that Beth played the white swan.

Both, of course. She was the prima ballerina, so she had to play both. In some adaptations of the Swan Lake, the white and the black swan are played by different baillerinas, but they are usually performed by the same person. Just because she appears as the white swan in the poster doesn't mean she only played that role. Nina also appeared as the white swan in the poster. Besides, Beth was known for her "magic" on stage, so she was able to play both roles and I totally see her dancing the black swan in her glorious years. Thomas keeps saying it was thrilling to watch her, yet dangerous. I guess he was refering to her dark side, which is what Nina lacks and the Black Swan required that evil side. I think Beth was perfect as the Black Swan, but she played both :)