I'm also suffering from this problem and I doubt this is some sort of bacterial infection or something similar to blackheads. I don't think this is a form of hyperpigmentation.
This thread also discusses about this issue.
http://www.medhelp.org/posts/Dermatology/Dark-line-at-middle-of-the-nose/show/768987?personal_page_id=1834201#post_6256966
Hello,
Horizontal line on the nose can be due to hyperpigmentation of the skin due to excess deposition of melanin or due to atopic allergies which causes the individuals to push up the nose. Treatment involves hydroquinone, azelaic acid, corticosteroids, tretinoin cream and laser treatment. Please discuss these options with your dermatologist before using them.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.