William Blake (1757 - 1827) was a real-life English poet who is frequently mentioned in The Mentalist, specifically concerning Red John. The two poems quoted so far that bear significance are "The Tyger" and "Cradle Song".
"The Tyger" first appeared in the episode "Red Sky in the Morning". The first verse was said by Red John to Patrick Jane after Red John saved his life.
"A Cradle Song" first appeared in the episode "Red Queen", when Gale Bertram quoted his last two verses.
A name of a Red John suspect, Brett Partridge, recall the title of his painting "A Brace of Partridge".
Another episode that remind Blake is The Great Red Dragon: Blake paints a series of pictures inspired at this character, by the "Book of the Revelation" (also known as "St. John's Apocalypse", from the Bible).