Earlier today, Apple release new iMac and now a software update has been released. The new update labeled as EFI Update 2.1 released and it address issues which cause a black screen to appear when installing Windows 7 or Windows 8 using Boot Camp Assistant.
According to release notes,
This update is recommended for iMac (late 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.