Ok, I think it might not be a boot problem, but a graphics card issue. I just restarted my machine and got the black screen again as it tried to boot, and this popped up:
"The system is running in low graphics mode -- your screen grpahics card and input device settings could not be detected correctly, you will need to configure these yourself"
After a few tries I got it to boot up, but my switchable graphics were acting up again. I found a script to disable switchable graphics at startup, but this time it ddin't work and I had to do it manually (my fans runs constantly and the machine runs hot if its not disabled)
The solution I found to disable it at startup called for placing the three 'echo' lines in --> gksudo gedit /etc/rc.local. I also added the fstrim -v / and another ssd option yesterday in the same place.
Anyway, the black screen at bootup might be a graphics issue related to the ongoing apport-gpu errors I've been having.
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
# In order to enable or disable this script just change the execution
# By default this script does nothing.
echo ON > /sys/kernel/debug/vgaswitcheroo/switch
echo IGD > /sys/kernel/debug/vgaswitcheroo/switch
echo OFF > /sys/kernel/debug/vgaswitcheroo/switch
fstrim -v /
# Modification for SSD
for dir in apparmor apt cups dist-upgrade fsck gdm installer samba unattended-upgrades ;
if [ ! -e /var/log/$dir ] ; then