Hi,
Greece is one of the EU member states which apply directive 2004/38 to its own citizens so to be in compliance with this directive as well as Greek law the embassy should issue your wife with a visa free of charge and follow a special fast-track procedure which is only allowed to take more than 2 weeks in very exceptional circumstances.
That's the legal part. Now to the REALITY part of things:
You're in West-Africa. You have married an AFRICAN. This makes things very, very hard for you since the EU embassies in Africa are dearly beloved, bigotted and completely ignorant of basic law.
First, you have to arm yourself with knowledge of the law. Here's a link to get you started:
http://eumovement.wordpress.com/eu-coun ... fo-greece/
Next, you have to print out the relevant parts of Greek legislation stating your rights to be accompanied by your third-country wife to Greece and enclose this in the letter to the embassy in Naijja where you demand they comply with EU and Greek law.
If they still refuse you, write a letter to the Greek foreign ministry pointing out the non-compliance with Greek law at the Naijja embassy and demand that they intervene on your behalf.. you are a GREEK CITIZEN and have rights in your own country in that capacity.
There's also the option of finding a reputable lawyer back in Greece who specialise in cases involving foreigners.
Finally, if nothing comes of all this you COULD try to contact the SOLVIT network but honestly, those buffoons probably never even heard about directive 2004/38
http://ec.europa.eu/solvit/
Good luck with everything. I learnt all this the hard way a year ago when I was in West-Africa and in your exact situation too..
regards,
MSH.