I'm sorry to say the status of "Mangrove Black Hawk" was recently reviewed and it is now considered a mere subspecies of Common Black Hawk, with a much more restricted geographical range than previously thought (only northern Pacif coast of South America, from Colombia to Peru. So, Central-american birds, including those from Costa Rica, are now considered to belong to only one subspecies, nominate anthracinus, both along Carribean and Pacific coasts.
See at this address:
http://www.globalraptors.org/grin/researchers/uploads/155/manbhbullboc.pdf
This argument has just been discussed in the Forum, here:
https://www.birdforum.net/showthread.php?t=148521&highlight=mangrove+black+hawk
HBW Alive has adopted this solution, just like many other recent bibliographical references, including f.e. last year book "Raptors of Mexico and Central America".
Thanks and greetings,
Igor Festari, Italy