Were you involved in some quite active sex prior to this happening? Six weeks is a little too long for bruising to disappear - unless is was severe, and almost black in appearance?
What you may have done is done some blood vessel damage and you have the remnants of the leaked blood that will eventually fade away ..
So, look at what was going on prior to this appearing - was it particularly vigorous? Have you had sex since, or have masturbated on a regular basis since then? You need to think about the regular pressures and strains you are placing your penis under - this will further delay the healing process.
If you think you took risks with your sexual practices then it may be necessary to get that checked out, although you would most likely have other symptoms of that … what you most likely have is blood vessel damage - this may take quite some months to clear away completely, or it is possible you may get this reoccurring in which case you may need to seek the advice of a Urologist …