I would either stretch out the transition even longer, or just switch 100% to the Orijen right away. It sounds like the stool is formed, just darker and more moist? If it is a little mucusy (just a little), that is okay, it is because your pup is just a little sensitive to change. If it is full-blown mucus, that is not normal.
I was feeding my two Orijen 6 Fresh Fish which was wonderful, but it turned out to be slightly too high in protein for my gang so I stepped down to Acana Pacifica (also rotate with Acana Grasslands) and they thrive on that. On a high quality food such as Orijen, you should notice that their poop is smaller and/or they poop less once they are fully transitioned to it.
I'm not sure what food you were feeding previously, but stick with the Orijen! You can give Acana a try if his poop doesn't firm up enough on Orijen.