From Dan Pascu on darcs-users:

What's even more confusing, is that if I specify both a patch name and a file 
name, the patch is completely ignored and I get the full annotation for that 
file (even if the filename I provide is one affected by the patch I specify):

darcs annotate --patch "Foobar" filename

I'm adding Dan and Tommy to this. 

Tommy wrote below...
http://lists.osuosl.org/pipermail/darcs-users/2009-August/020845.html

Tommy: do you think we can close this bug then?  Perhaps updating the
documentation along the way?

On Wed, Jul 15, 2009 at 02:25:21AM +0300, Dan Pascu wrote:
> What's even more confusing, is that if I specify both a patch name and a 
> file name, the patch is completely ignored and I get the full annotation 
> for that file (even if the filename I provide is one affected by the patch 
> I specify):

No, with both a file and a patch you will get the contents of
that file at the time that patch was applied (in inventory
application "time" order), including what lines where removed
from it. Unfortunately the output format verges to
human-unparseable.

It could be this information is more valuable when developing
and debugging patch types to see their exact effects and how
they commute, then to examine the repository's past.

Now that annotate is only about annotating files (and not showing patch
contents) there should be no more confusion.