It might be worth clarifying that "Comcast" proper is not buying Blackstone's 50% interest. NBCUniversal is purchasing the 50% interest, giving them full 100% ownership of the Universal Orlando Resort.
The reason all the articles mention that Comcast is purchasing Blackstone's interest is because of Comcast's recent purchase of controlling interest in NBCUniversal. Since Comcast just laid out a TON of money in that deal, and they have control of NBCU, they ultimately had a lot of say in the decision (and may even be providing funding to the subsidiary to make the purchase).
It also makes for more interesting headlines to say that Comcast is purchasing Universal Orlando, than to say NBCUniversal is purchasing the other 50% of Universal Orlando.