Beloslava was daughter of tsar Ivan Asen II and his first wife Anna (religious name Anisia), mentioned in the Synodik of the Bulgarian Church.
It is possible that Beloslava and her sister Maria were illegitimate children of Ivan Asen because his first marriage with Anna was not recognized by the Bulgarian Orthodox Church.
The marriage was arranged by his uncle Rastko Nemanjić in order to secure good relations between the Kingdom of Serbia and the Bulgarian Empire.
Bulgarian political influence in Serbia ended after the death of tsar Ivan Asen II during the invasions of Tatars in Western Europe.
The new Serbian king insisted that she should be kept under strict control and, in response, received an oath, in writing, stating that Beloslava would not be allowed to return to Serbia.