This is a question that I am asked about constantly, and it is really simple: Edit the file /usr/X11R6/lib/X11/app-defaults/XTerm and add the following:
XTerm*background: black
XTerm*foreground: white
I usually do it after the savelines option. Most other names work, too (yellow, cyan, etc). For more info, you can check the manpage on xfree86.org and Thomas Dickey’s xterm site.