I use Autoglym and I'm always impressed with the result of the annual polishing.
I use their normal polish, then their one for modern paints, then the paint protectant. Normally takes me the best part of a day to do.
Looks good on trophy Blue, Starlight Silver or Goodwood Green. Cannot comment on Black though, and I have no experience using it on black cars.
Could some grit have landed on the car when you were polishing?