Yes, the master configs will be set up in a way that any mod that any pack uses will be rank and file in it's own designated item ID. Not too sure about mods exclusive to the 3rd party packs. (e.g BnB, AS, MF2, DW20)
You only get problems when you add mod that isn't originally in any of the FTB packs and or you are useing a later version of a mod that has added an item or block.
For e.g Recently Reika added a potion to Rotarycraft v22, FTB packs seem to be using v19, the potion was fighting over Ars Magica for an item ID. Those that upgraded RoC to v22 found that their game wouldn't start due to a small ID conflict but both those mods are present in standard FTB packs.
So yes open blocks is in one of the offical packs so it should slot in with other official packs, but be mindful of newer version of mods that add items/ blocks. You may need to be prepared to resolve ID conflicts.