Before you do that remove the Slackware package first:
# removepkg blackbox
I'm guessing that doing ./configure, make, make install will install it to /usr/local, so it wouldn't matter. But to be on the safe side, and to save yourself from potential confusion, you better remove the Slackware package first. Also, you can probably use the SlackBuild script for blackbox to build 0.7.1.