Firstly, within Blackbird:

1. Save your sequence.

2. Drag the saved sequence into the AAF 'blue' publish button. Move the sequence (marked as 1) onto the AAF symbol (marked as 2)

3. Wait until the sequence shows as "Published".

4. Double click on the published sequence (this will open a webpage with export details)
There is a Download AAF option in the center column, which will allow you to download the AAF file. 

If you're still having trouble please contact us at support@blackbird.video and one of our Support staff will then contact you with suggestions on how to resolve this issue.