The crossroads is a unique place in that it is a place that is between places (neither this way nor that way), and it is also a directional point from which decisions or directions must be made. Consequently, there is common association of crossroads to special deities or spirits found in many cultures - including West African cultures.
The Black Man at the Crossroads
In hoodoo, there is a traditional ritual performed in order to gain mastery over some manual skill - typically playing guitar, playing dice or singing/dancing. The most famous example of this is typically attributed to Robert Johnson who is said to have "sold his soul" at the crossroads to learn mastery at writing songs and playing the guitar. This attribution is a mistake, however, and it was actually his contemporary Tommy Johnson (no relation) who said this and performed this ritual. When asked, Tommy Johnson stated,
"If you want to learn how to play anything you want to play and learn how to make songs yourself, you take your guitar and you go to where a road crosses that way, where a crossroad is. Get there, be sure to get there just a little 'fore 12:00 that night so you'll know you'll be there. You have your guitar and be playing a piece there by yourself.... A big black man will walk up there and take your guitar, and he'll tune it. And then he'll play a piece and hand it back to you. That's the way I learned to play anything I want." (As told by LeDell Johnson to David Evans and quoted from Peter Guralnick's Searching for Robert Johnson, copyright © 1982, 1989.)
It is also important to note that the man encountered at the crossroads is black - as in the color black - not of African descent. This can be interpreted to be dressed completely in black, or that his skin is all black in color, etc.
The Crossroads Ritual
Accounts on this ritual vary a bit. Some folks say to go for 9 consecutive nights, others say 3. Some say to go before dawn and wait for the sun to come up. Here is my personal suggestion:
Perform this act for 9 consecutive nights. Take the item you wish you master with you to a crossroads (where two streets intersect at right angles to make a "+" shape) arriving about 15 minutes before midnight. When you arrive, sit on the corner and use your item (toss your dice trying to get a 7, play your guitar, etc.) the entire time you are there. Continue until about half an hour past midnight. While you are there, take note of black animals that might approach you, people dressed in black, etc. There is absolutely no need to do anything aside from showing up and doing your skill that you wish to master. The effort of showing up every night IS your offering to the spirit of the black man at the crossroads.
On the final night, a black man will approach you, take the object from your hand, show you how to use it, and hand it back to you. You've just made your pact with the Black Man at the Crossroads to become a master in your skill.
Some folks say that you are selling your soul to the Devil. Others say that you're making an alliance with him. My own personal experience has shown me that the spirit of the Black Man comes with you and inspires you in your work from that point forward, but your experience may vary.