Trying to make black transparent, and export with alpha.

Hi,

Apologies for not getting the terminology correct here.  I have a short looping clip of some Christmass Holly.  The centre of the image is black, and I would like to make the black be transparent and export that to use in a media server on top of another layer.

Exporting isn't a problem, I'm just not sure how best to make the balck transparent.  I have been experimenting with Set Matte, and Alpha From Lightness.  This works to a degree but is also makling my green holly transparrent.  Can anyone point me in the right direction?

Thanks!