I'm wondering how easy it would be to convert Blackmoor MMRPG over to the same timeline as Living Greyhawk.

I realise that Dave Arneson's Blackmoor wasn't designed to be placed into the Flanaess, so some degree of would be required, but it seems to me that the Blackmoor MMRPG episodes (along with the adventures that are folded into the seasons) make for a well organised pre-made long-term campaign.

And if Blackmoor MMRPG was used alongside Living Greyhawk, the core LG documentation (and things like the Living Greyhawk Gazeteer) might be able to fill in for any elements of Dave Arneson's Blackmoor (or the Blackmoor MMRPG) that are not appropriate for the Blackmoor within the Flanaess.

Does anyone know of anything in the 3rd Edition Dave Arneson's Blackmoor line or the episodes of Blackmoor MMRPG that would be especially problematic?

Most of the adventures deal with things local to the Blackmoor region, so if you assume that Blackmoor at this time is similar to how it is presented in the ZGG line you should be okay. For things like the Hadeen arc, you would need to decide if you want to incorporate Blackmoor deities or simply replace them with Greyhawk equivalents.

I cannot recall any specific adventures dealing with the Duchy of Ten, but here you will have problems with the fact that the location of the Duchy of Tehn is different corresponding to Greyhawk's Blackmoor than in the Arneson version.

-Havard

Sort of laterally related to this would be linking up the OD&D Castle Greyhawk campaign (using the old Avalon Hill Outdoor Survival Board) with Blackmoor. You could consider the board to be the area beyond the borders of the Barony and its surrounding environs.