After its fall to the Avars in the early 7th century, the ancient city ceases to be mentioned, and its fate on the subsequent centuries is obscure.
The Slavic Beligrad, the "white city" (named for the color of the stone it was built from), had been established by the 9th century on the site of Singidunum, within the First Bulgarian Empire. It is first mentioned in a letter written on 16 April 878 by Pope John VIII to Bulgarian prince Boris I Mihail. With its new name, Beograd would eventually be restored to its earlier strategic significance.
This is my version of Singidunum.