I know you can auto-lock Ubuntu MATE when suspending, but I run BOINC and I'd like to lock the screen and stay running. I tried the screensaver, but that blanks the screen but screen still shows a sight purple hue, so it's not actually off. Right now I have the screen blank set to 10 minutes, but would also like it to lock, similar to regular Ubuntu and the gnome-screensaver, which performs this function.
Not quite sure what you are looking for, but have you tried:
System -> Preferences -> Hardware -> Power Management
There is a choice for Display - Put Display to Sleep When Inactive -> (Choose minutes)
Thanks for your comment @mdd12 That is what I have selected. I also want to lock the PC either at the same time or whenever. I don't want to use the screensaver blanking as the backlight stays illuminated with a kind of purple tinge.