When one of these engines won't start after sitting for 30+ minutes, it's usually due to one of a few items (not necessarily in any order):
1- Poor fuel atomization.
2- Slow cranking speed.
3- Low cylinder compression.
4- Injection timing.
5- Cam timing.
and maybe others... always begin your troubleshooting by eliminating the cheapest/easiest possibility first.

Generally, these engine will start normally without glow heat down to less than about 80 degrees F. As an aside... Sarah's 1994 6.5TD Blazer is running an aftermarket PCM chip. This chip glows the glow plugs hot or cold. It's either a Heath or Kennedy chip.