The muzzle does look a little long. Could be a maltipoo. I've seen ones that look similar. I think you should DNA test and let us know!i'm thinking Havanese. The muzzle is pretty long for a Shih Tzu or a Maltese. I googled "black Havanese" and it pulled up pictures that looked a lot like this little one. Of course I think they usually are larger than either Maltese or Shih Tzus.
I had a cockapoo who looked like this also! She was the runt, the rest of the litter were curly like the poodle mom. I help with rescue here and we had a Maltese mom (looked full blood) and all of her pups were black! Tiny like her but 2 had a small white spot, but all black. Father unknown but they looked like Maltese puppies, except black!My first dog was a cock-a-poo & it looked just like this.