Why do you believe the official spelling is AzulOscuroCasiNegro?

I checked a few sources, and it looks like they are both used:

https://web.archive.org/web/20120710081041/http://www.altafilms.com/site/sinopsis/azuloscurocasinegro
https://web.archive.org/web/20060407092613/http://www.tesela.com/azuloscurocasinegro.html

Thanks for the answer!

Actually, those two sources show the title as I tell you. Another reliable source it's Filmaffinity, probably the most active movie website and forum here in Spain. The entry for the movie is here: https://www.filmaffinity.com/es/film536424.html

Also, if you see the poster of the movie they play with standard/bold letters to remark each word (they don't use upper cases for aesthetics, as they don't even put it at the beginning). The common way to refer to the movie here is like that, AzulOscuroCasiNegro.

if you see the poster of the movie they play with standard/bold letters to remark each word

Yes, I noticed. I'm assuming the opening title is similar?

You mean in the movie itself? I've checked now my copy of the movie and it doesn't show the title, which is weird. Not at the beginning nor at the end. In the trailer meanwhile it appears as in the poster: https://www.youtube.com/watch?v=XERzdtk-FY0 (end of the video).

Thanks for checking. That it pretty weird... but I've updated the original title.

It is indeed. Thanks for the attention!