Song of the Gringo
This early Western, directed by John McCarathy, was the film debut of the "singing cowboy" Tex Ritter (father of the late TV actor and comedian John Ritter), in which Tex is sent to infiltrate a gang of claim jumpers.
L to R:
Joan Woodbury as Lolita, unidentified Newfoundland, Tex Ritter as Tex, Martin Garralaga as Don Valle
A black Newfoundland is shown in two extended scenes early in the movie (and not again after that); in the film, the dog belongs to Lolita (played by Joan Woodbury), the daughter of the Spanish don whose estate is used by the criminal gang of claimjumpers, and at one point is cited as evidence by the gang's leader that Tex is spending a lot of time hanging around the don's beautiful daughter, for the dog (who is seen sitting by Tex as he fixes a horse's shoe) is said to only associates with people who spend a lot of time around him and the don's daughter (although a moment later the dog greets and then accompanies a likeable but clueless gang member, so maybe the dog's not quite that particular after all).
The first scene with the Newfoundland begins about 18 minutes into the movie, and the dog is onscreen for perhaps a couple of minutes total throughout this extended scene, lying near or sitting by the don's daughter, even at one point seeming to pay attention to Tex's singing.
The second scene featuring the Newf immediately follows the first. The dog, onscreen for a total of about 30 seconds, is seen sitting near Tex as he shoes his horse before leaving with one of the gang members.
This movie is available on Amazon Prime, though there is also a (free) version available, in not-particularly-high video quality, on YouTube [Western Mania].
Here is the movie's IMDB.com page