Is it normal for an external HD to blink its light and still be visible in Disk Utility, after it's been Ejected?

ForeignerMullet wrote:

After Eject, the drive disappears from Finder, stops spinning and its light starts a slow blinking pattern,

This is probably normal. Some drives blink when being written to/read from, while others blink unless read/write is occuring.

but in Disk Utility the volume appears greyed-out and unmounted while the HD is still visible (not greyed-out). If you try to re-eject it from there nothing happens...

If this is just for a few seconds, that's normal. If it's permanent, it's not.

Disconnecting the HD's USB cable after a normal Eject (from the Finder), does not prompt any type of "incorrect ejection of volume" messages.

Normal/good.

Other drives that I tried, only differ in the activity of their light post-ejection - some stay still, others blink.

See above.

Just for reference, when I eject a USB Pendrive from Finder, it disappears completely and stops blinking.

Normal; a flash drive has no source of power and could not possibly keep blinking.

What is the exact make & model of the external HD?

Depends on the drive and the interface. I have a WD My Passport 2TB that blinks all the time, and an Oyen Digital 750GB that almost never blinks. And while connected, both will always show up in DU.

The key point is that the drive NOT appear in the finder windows or desktop, to ensure that all files got closed and the volume got unmounted, prior to disconnect. Else you risk data loss. Furthermore, the modern Lions are excessively shy and don't like showing their stuff. Be sure to open Finder's Preferences and confirm that the External Disk items are enabled in both the General and the Sidebar panes. Else you won't know that the drive was not unmounted properly. Default is not to have these items checked.