Black spots in basin waste.

[icelollycraving]

I've had a major blitz today. The ensuite is used all the time & is cleaned thoroughly. Whilst cleaning out the shower trap thing & the basin,I noticed some black stuff too far for me to reach.
I've tried descaler,vinegar,bleach & it's still there. I tried all seperately with plenty of boiling water. Any ideas please?

I've tried bathroom sink unblocker,limelite as well now. It's removed a little bit but not all. I can't get at it & it's driving me mad!