Just a note....I enabled the Context 3.0 module while using Arthemia 2.0 as my 'default' admin theme on my D6.22 installation. While creating a new context, I tried selecting a 'Block' reaction in the 'Reactions' section of the Context admin form (/admin/build/context/list//edit) and found that no blocks were showing up for selection in the right-hand selection area. I found that changing the admin theme to garland fixed the problem.

If you are having this problem, I hope this will help you resolve it. Try changing the admin theme to garland or some other theme and hopefully this will fix this issue for you

Best,
Gary