According to the New York Times, he apparently died of a heart attack.
Boelzner was with NYCB for over 45 years. He joined the company as a rehearsal pianist and went on to conduct and play solo piano, performing at the premieres of Robert Schumann's "Davidsbundlertanze" by George Balanchine, with whom he had a close working relationship, and Jerome Robbins' Dances at a Gathering and The Goldberg Variations.
In 1990, he became the company's music director, and remained in that position until 1999, when he retired. He stayed with the company in the capacity of music advisor until his death.