I cut my teeth on Beulah Land; I was one of those babies sleeping on the front bench while my father preached. At about 9 years of age I read John Bunyan's Pilgrims Progress; here's a snippet:
"Now I saw in my dream, that by this time the Pilgrims were got over the Enchanted Ground, and entering into the country of Beulah, whose air was very sweet and pleasant, the way lying directly through it, they solaced themselves there for a season.
Yea, here they heard continually the singing of birds,and saw every day the flowers appear on the earth, and heard the voice of the turtle in the land. In this country the sun shineth night and day. wherefore this was beyond the Valley of the Shadow of Death, and also out of the reach of Giant Despair, neither could they from this place so much as see Doubting Castle.
Here they were within sight of the city they were going to,also here met them some of the inhabitants thereof; for in this land the Shining Ones commonly walked, because it was upon the borders of heaven. In this land also, the contract between the bride and the bridegroom was renewed; yea, here, As the bridegroom rejoiceth over the bride, so did their God rejoice over them.
Here they had no want of corn and wine; for in this place they met with abundance of what they had sought for in all their pilgrimage. Here they heard voices from out of the city, loud voices,saying, Say ye to the daughter of Zion, Behold, thy salvation cometh! Behold, his reward is with him! Here all the inhabitants of the country called them, The holy people, The redeemed of the Lord, Sought out"
Beulah appears in the Bible once; speaking of Jerusalem Isaiah wrote in 62:4:
"No longer will they call you Deserted, or name your land Desolate. But you will be called Hephzibah (King Hezekiah's wife), and your land Beulah; for the LORD will take delight in you, and your land will be married."
Blake drew on Isaiah and John Bunyan for Beulah; this is only one of many, many sources in the Bible that he used (for his own purposes!).