When you first boot Slackware after installing, it stays in text console mode. I think the problem you have is the frame buffer graphics mode is not compatible with your video card.
You can try adding these kernel parameters in the grub menu entry for Slackware on the line that has "/boot/vmlinuz".
Code:
vga=normal video=640x480
Edit the "/boot/grub/grub.cfg" file in the Ubuntu partition (or whatever partition has GRUB installed).
The menu entry for Slackware will look similar to this. Your GRUB 2 menu entry may look different and there may be a few more options on the line with "/boot/vmlinuz". Add the "vga" and "video" options to the end of that line. If the "vga" or "video" option is already there, then change the existing options.
Code:
menuentry "Slackware" {
recordfail
insmod ext2
set root=(hd0,msdos2)
linux /boot/vmlinuz root=/dev/sda2 ro vga=normal video=640x480
}
If you still do not see a display, you may have to black-list the Noveau graphics driver.
Create a text file in the Slackware partition named "/etc/modprobe.d/disable-nouveau.conf". The file should contain this to disable Noveau.
Code:
blacklist nouveau
options nouveau modeset=0
For example, if you mounted the Slackware partition under "/mnt" then you need to create a file called "/mnt/etc/modprobe.d/disable-nouveau.conf". The directory "etc/modprobe.d" should already exist in the Slackware partition.
If that does not solve the problem, please post information about your graphics card. Use this command on a version of Linux that does boot and post the results here.
If you know the manufacturer and model of your graphics card or chip, post that as well.