I obviously don't know for sure, but JKR was pretty exacting in the details she put into the HP series in general, so I err on the side of Mrs. Black making as many appearances as she needed to in Deathly Hallows. Personally, I don't think she was softer than in Order of the Phoenix. From Deathly Hallows:
Hermione screamed and so did Mrs Black, her curtains flying open. . . [SNIP] ‘Mudbloods, filth, stains of dishonour, taint of shame on the house of my fathers –’
They made so much noise that they woke the portrait of Sirius’s mother as they passed through the hall. ‘Filth! Mudbloods! Scum!’ she screamed. . .
He had forgotten the portrait of Mrs Black: at the sound of his yell the curtains hiding her flew open and she began to scream, ‘Mudbloods and filth dishonouring my house –’
He waited until he was halfway down the stairs to the kitchen, out of earshot of Mrs Black and clear of the dust cloud before calling, ‘I’ve got news, and you won’t like it.’
Deathly Hallows - British Hardcover - Chapters 9, 10, 11
Although Molly Weasley uses stunning spells on the other portraits in the hall in OOTP when Harry first encounters Mrs. Black, she uses her hands to try and tug the curtains shut. Likewise, Lupin and Sirius manually wrestled with and eventually closed the curtains.
Presumably magic would be more powerful than trying to manually deal with Mrs. Black's portrait; this would explain why Mrs. Black's portrait is more easily dealt with in DH than OOTP. Harry, Ron, and Hermione use magic against the portrait in DH; I cannot find an example in OOTP where anyone uses magic against Mrs. Black. Definitely correct me if I'm wrong.
J.K. Rowling has described portraits as displaying certain characteristics of the person, but that they are limited in thought and tend to stick to "catchphrases" they used while alive. Hence Mrs. Black being so two-dimensional and her repetition of basically the same phrases. She is just a portrait. She's not any more powerful than any other portrait, so eventually taming her curtains isn't far-fetched.
Mrs. Black's portrait was alone in the house with Kreacher for at least ten years, which means her portrait wasn't maintained or kept clean. Her curtains are described as moth-eaten. Perhaps the length of time the portrait went without care caused some kind of deterioration that made the curtains difficult to open and close.