The EU Blue Card is a residence title as defined in the German Residence Act (§ 19a AufenthG). Directive 2009/50/EC of the European Council of May 25, 2009 constitutes the European legal basis for the EU Blue Card. The EU Blue Card is the central residence title for academic professionals from countries outside the EU for the purposes of taking up highly qualified employment in the EU Member States. In addition to a university degree, a contract offering a minimum salary is required to obtain the card. The card is initially granted for four years or, in the case of shorter employment contracts, for the duration of the contract plus an additional three months.

Source: Bundesministerium des Innern, AufenthG, edited (only available in German)