Today I finally installed Windows Mobile 6.1 on my BlackJack II. The process is fairly straight-forward and explained pretty well at this link:
BlackJack II – Windows Mobile 6.1 Upgrade Instruction

Be sure to follow the instructions EXACTLY and do not skip any steps like I did. For example, be sure to install the modem drivers, or you will lose your connection to your device midway through the install. I overlooked this step and almost caused me to have a heart-attack because I thought I had hosed my phone.