Apologies - I did test - maybe prior knowledge does not help. Dearest Brichey - I appreciate you've chosen a name that rhymes with 'bitchy" but do try and rise above such instincts.
When you test, you have to assume absolutely nothing: just follow logic and logic alone. Think of every possibility for the black squares and only fill them in there really is no chance that they are blank. After a while, you'll learn what sorts of things make it illogical.
No worries! One time, I had to test one of my puzzles a second time (without any alteration) because I wasn't sure if I'd made an assumption or not! Wasted 15 minutes of my time!