A device for blow molding containers. Preheated preforms made of a thermoplastic material are transferred within a blow molding machine to a blowing device. In the region of the blowing device, the preforms are shaped into containers by the blow molding by a blowing pressure. In the region of the device for blow molding containers, a blowing nozzle seal is arranged, which is brought against a support ring of the preform. The blowing nozzle seal is formed by a sealing element and a support sleeve. The blowing nozzle seal is first mechanically preloaded in relation to the support ring after contact with the support ring and then is further loaded pneumatically. An accommodating space that surrounds the mouth section of the preform is thereby provided, which accommodating space has substantially the same pressure as the interior of the preform in order to avoid deformation of the mouth section.