Blomberg was born in Regensburg, (modern Germany), the eldest daughter of Wolfgang Plumberger or Blomberg, a burgher, and of his wife Sibilla Lohman. A singer, in 1546 she was for a short time the mistress of the Holy Roman Emperor Charles V, who was in Regensburg for the meeting of the Imperial Diet. On 24 February 1547 Blomberg gave birth to John of Austria, who was almost immediately taken from her and sent to be raised in Spain.