Carbon black, also known as lamp black, bone black and thermal black, is a hydrocarbon compound with a small, round molecule which is both darker and more likely to migrate under the skin—hence the appearance of very old tattoos which have lost their sharp lines.  Iron-oxide black, generally used for permanent makeup, has a larger, flat-sided molecule which is less likely to migrate, though it doesn’t appear quite as dark.  One solution for darker blacks is to add a blue compound (copper pthalocyanine blue)—hence the bluish appearance of some very old tattoos.  I use a little purple to create the blackest eyeliners for my clients who request them, but most eyeliners are a very dark brown with a drop of black added.

This information is courtesy of the Society of Permanent Cosmetic Professionals (SPCP).  Please visit www.spcp.org for additional information.

In answer to the question of pigment and MRI safety, I am not aware of any actual cases of MRI difficulties, and  I quote from my professional organization’s website (www.spcp.org):
” According to Dr. Frank Shellock of Tower Imaging in Los Angeles, CA, a top expert in MRI safety, [out of the thousands who have had permanent makeup applied], only a handful of people have reported minor problems around the eye area and no problems around the lip or brow area. … Test studies have confirmed that the ‘iron’ particles in pigment are too microscopic to react as true metal pieces but rather are more accurately compared with ‘metals’ which already exist microscopically in the body.”