Cast iron pan has black residue on it
You may notice the black residue if you fry eggs in the pan as well.  

Most likely, the black residue is charred (greasy) food sticking to the seasoned oil.  Since Flaxseed oil has low heat tolerance, it could be that disintegrating, too.  Otherwise it could be related to the iron in the cast iron which isn't bad for you (some say even good).

a) Is it normal: Yes if you keep with the same regiment (recommend you don't).

b) What is Wrong:  Water won't perfectly wash non-polar chemicals such as burnt solids in grease (milk solids while making ghee), so the hot water no-soap routine leaves the stuff in pan.

c) How can I fix it: Try the following going forward:


After you finished cooking and while the pan is somewhat hot, put a heap of salt in the middle and with a paper-towel spread and lightly scrub the pan.  The salt will pick up the would-be black stuff, largely disinfect the pan, and the abrasive properties of the salt helps with the cleaning.
Wipe off the now brown salt from the pan and rub-in the coat of oil as you normally do. The new coat will stick better to through the salt's abrasive effect.


This routine won't eliminate residue altogether.  But it seems to work great long term.

Note on oils:
Flaxseed oil is best consumed cold and within three weeks.  I haven't heard of a credible source promoting seasoning the pan with flaxseed oil as it has one of the lowest smoke points in the oils.  Use a more heat tolerant oil like light (not extra-virgin) olive oil or canola.