Blue Shift originally came with a High Def model pack that gave a lot more detail to creatures and weapons.(even if they did take a number of artistic liberties) Steam included that High Definition Pack, but installing it blocks all custom models in Half Life, Opposing Force and Blue Shift. It also apparently uses some bugged models that cause a number of scripted sequences to stop working, both in the original games and gold source mods as well.
Planet Half Life put together a fixed version that could simply be dropped into the Half Life and Opposing Force folders to replace the regular models with the updated ones, but Blue Shift handles models differently and makes it impossible to drop in custom content.
That Blue Shift: Unlocked creates a Half Life mod using the information from Blue Shift. This mod can then have the updated models from Planet Half Life dropped into it and it will work just fine. however it is still a goldsource mod, NOT a source mod, which is what Half Life: SOURCE would be compatible with.