Mac Mini - Installing Windows 7 from external CD/DVD drive - hangs before installation with black screen
this link identifies an black screen issue when installing Windows 7 on some iMac's, it may be relevant even though you have a Mac Mini

this link is the Apple FAQs for Installing Windows 7 in Bootcamp, there are some good resources there for the main issues most users meet.

this link is the Apple Install Guide for Windows 7 which I think you have already been following, but I'm linking it here for other people if required.