Your phone needs to be flashed, it can be done using SP Flash tool.
You need the stock firmware for your specific model of your phone. You can locate the model info behind the battery or at the back of the phone. Here is a link to help you in your search for the firmware.
Extract SP Flash Tool to a new folder, and extract the firmware to another new folder. Plug the phone powered off into the USB and MS Windows will install the driver automatically. If not then you will find the driver install info in the link to the BLU Google+ community. After drivers are installed unplug the phone.
Click "Scatter-loading" and locate the scatter file in the firmware directory you extracted. Change "Download Only" option to "Firmware Upgrade", do not choose "Format All + Download" as this would remove your IMEI number and you won't be able to use network services until its restored.
Click the green download button, then connect the phone to the computer. The flashing process will begin and the progress will show at the bottom. Once its done remove the battery for a few seconds and reinsert. Power on and all should be well again.
Should look something like this below.. If you need any further help please post the model of your phone and I'll provide more specific links.