If it still won't connect to a PC try the following.
Try starting the phone in safe mode to see if it will connect OK to a PC.
Not quite sure with a Blu phone but with most Android phones to get into safe mode, turn the phone off by pressing the Power button. When the shutdown menu options appear press and hold the Power Off option. There should be an option to start the phone in 'safe mode' Tap that and allow the phone to restart. Once in safe mode see if you can now connect to a PC.
If you can then a downloaded app is the cause of the problem.
The trick is to find out which one. You need to uninstall each downloaded app one by one and test the phone between each uninstall to see if it now works OK. If it does the last app that you uninstalled is the culprit.
To get out of safe mode, simply turn off the phone in the normal manner and restart it.
If it still won't connect to a PC in safe mode, try a hard reset.
Be aware that a hard reset will erase all your data and downloaded apps. It will restore the phone to its factory default condition, (or last system updated condition).
Perform a backup of the phone, using the phone's backup feature first, before you do a hard reset.
Then perform a hard reset using the phone's factory restore feature.
Once the phone has been reset, try connecting to a PC via USB.
If it works, restore the phone with the backup that you made earlier, using the phone's restore feature.
Once it has been restored, check if you can connect to a PC via USB.
If you can then the reset has resolved the problem.