Ive actually solved this problem myself, however i thought id share it for other peoples benefit.
standard install of mageia 6 64 with nvidia GT610 hardware. manufacturer is asus if that has any bearing. (im sure this issue will effect other nvidia hardware.)
Im not reporting this as a bug, as you will see its already fixed.
when installing mageia 6 , i always take the option to update before the DVD installer reboots, and everytime since mageia 1 the same issue happens with the DVD installer, that is the first rpm, glibc, bits get updated, then on restart i get dbus error, dbus not running , and the second part of the upgrade fails, and i cant believe im the only one seeing this , or is it something about the way i install , (suspect it is related to https://bugzilla.redhat.com/show_bug.cgi?id=1271551 but have not investigated it, as its not an issue). i dont know, but off topic for this tale.
so i then reboot and update after the first boot, and believe me when i say ive installed hundreds of mageia x's over the years.
anyway this particular one: black screen on startup. rebooted just in case it was a quirk, but no its a hard and fast problem.
so I hit control-alt-f4 and login root/password
urpmi --auto-update (its mostly got the media from the run in the installer, only missing the tainted stuff and thats not relevant to the nvidia driver)
when thats finished reboot, black screen gone, and plasma login is there.
so obviously this is a bug with some nvidia cards in the current 6.0 release media (6.1 will undoubtedly fix this as it will come with a newer NV driver, but thats not to say some NV graphics cards wont have this issue on that, so its a good first place to start.
its possible this has been reported already, but since ive just found it and not everyone looks through the bugs i thought id add it to the forum for those that use search.
regards peter