Thank you Terri.
One of the ways to do this sketch would be to use a number of pens delivering lines of varying line thickness, and darkness. But I have used a single pen with a flexible nib (flex nib) to vary the thickness of my lines. And, as you rightly guessed, I used the hard tip brush pen to draw the very dark black patches. I could have used a conventional brush pen (soft tip) for that, but I only had the hard tip brush pen. I could also have done it with an ordinary brush dipped in ink. And of course it could have been done by close cross hatching with even the flex pen that I used for the rest of the sketch. But I wanted the effect that you see in this sketch.