I do not know the answer (BTW, telling which video card you have and video driver you use could help investigate).
Meanwhile, you could disable completely console blanking, if you don't need it.
To do that edit /etc/rc.d/rc.M, and comment out this line:
/bin/setterm -blank 15 -powersave powerdown -powerdown 60
or change the parameters.
PS Maybe the setting has changed a bit, I have that line on Slackware 14.0. HTH anyway.