Apple released MacBook Air EFI Firmware Update 2.7 for mid-2013 MacBook Air models. The update addresses issues with installing Windows 7 and Windows 8 in Boot Camp, as well as another Boot Camp issue.
Apple's patch notes:
This update is recommended for MacBook Air (mid 2013) models.
This update addresses an issue which may cause a black screen to appear when installing Windows 7 or Windows 8 using Boot Camp Assistant if both an external optical drive and USB thumb drive are connected to the system. This update also ensures that the system will boot by default into OS X after installation of Windows 8.
You can download (4.8MB) the firmware update directly from the KnowledgeBase article. It should be available in the Mac App Store under Downloads, as well.