Beulah is a name given to Jerusalem in the Bible, and refers to the Jews returning from exile and God being pleased with them:
"No longer will they call you Deserted, or name your land Desolate. But you will be called Hephzibah [my delight is in her], and your land Beulah [married]; for the LORD will take delight in you, and your land will be married." (Isaiah 62:4)
Beulah is also mentioned in Bunyan's Pilgrim's Progress, where it is described as a land of peace where 'the sun shineth Night and Day' and the 'air was very sweet and pleasant'. By suggesting to Christabel that 'Richmond is Beulah', Blanche is clearly saying that their home is peaceful and happy, and perhaps that God is pleased with them. But since Beulah also has the meaning 'married' it also hints at the homoerotic feelings that Blanche supposedly has for Christabel.