If you still need the Stock firmware let me know. I had the same issue and just fixed this phone by flashing BLU_D850Q_V28_GENERIC_DCC with sp.
These are the instructions I used however the link to the rom in that manual did not work for me but somewhere I found one that worked. http://firmwarerom.com/blu-d850q
I am not sure where I actually got the firmware but I have the firmware on my computer as I did this ten minutes ago and can upload it to drive or something if you still need it.