Is HALLOWEEN just a BLACK CHRISTMAS rip off?

Yes. 

As much as I love John Carpenter's HALLOWEEN, I must admit, it does copy, practically beat for beat, BLACK CHRISTMAS. Such as:

• Similar title
• Opening scene POV
• Obscene phone calls
• Final girl trope
• The villain escapes at the end

Halloween 1978

Carpenter and his team likely saw the success BLACK CHRISTMAS had by using a title that exploits a popular holiday, so they did the same. Except, they were smarter and used a holiday that already evoked horror elements. 

The opening scene of Billy climbing up the house and making his way inside the attic is improved in HALLOWEEN. It's basically the same point of view style, but Michael's intentions are different. He goes inside the house to kill his sister. Writers Deborah Hill and Carpenter took BLACK CHRISTMAS' opening and tweaked it just a little to make the opening message more exciting.

BLACK CHRISTMAS is known for its weird phone calls. In fact, the whole movie uses phones as a plot point. The big reveal of the story is when the police discover the calls are coming from inside the house. HALLOWEEN uses phones, but not to such a big degree. 

The final girl trope, as we know it, was really created by THE TEXAS CHAINSAW MASSACRE. A screaming, completely hysterical female lead who narrowly escapes the male killer is something the slasher genre is known for. Laurie Strode, perhaps the most famous final girl ever, played by the legendary Jamie Lee Curtis, is exemplary of this trope.

Famously, in BLACK CHRISTMAS, Billy gets away with his crimes, still hiding in the attic with two dead corpses. HALLOWEEN uses the same trope. Michael Myers eats multiple shots from Dr. Loomis and falls off the balcony to his presumed death. It's left ambiguous. 

Now, all this being said, what makes HALLOWEEN more enjoyable is the characters. In BLACK CHRISTMAS, for the most part, each character is forgettable. There are too many sorority sisters, and they all act the same. HALLOWEEN learns from this and reduces its characters to just a few. By doing so, it allows Laurie, Lynda, and Annie more time to shine in their characters. The dialogue in HALLOWEEN is very good, too. 

But perhaps the biggest and best upgrade comes from the villain himself. Whereas Billy is never seen, just an eye here or a hand there, Michael is unmasked in the opening. From there, we see him as a masked killer in full-frame shots. The viewer understands that Michael is there. He is present. He's not just hiding behind a door. He has no problem venturing after his victims. 

So yes, HALLOWEEN does copy BLACK CHRISTMAS to some degree, but it improves on it at the same time.