Dear Xavier,

I have no experience with Amira or Blender, but a colleague told me that it is possible to export Amira files to .obj or .stl instead of .am and that Blender can import .stl or .obj.
See here for Amira documentation: http://avsl.cct.lsu.edu/vizclass_illustration.html#save-individual-surface-and-export-to-desirable-format
I hope it solves your issue.

Best regards,
Marion