If you recently upgraded to a Moto X4 (released 2017) then I can only imagine how old the Blu phone was. I have used Blu in the past as a backup phone for when I was getting my primary phone repaired, I as well was not happy with it’s performance. You are pretty much getting what you pay for in comparison, currently their top of the line phone is just over $200.00. To keep their prices so low they use slower older hardware, even though they are running they latest version of Android.
So to finally answer to your question, the issue seems to be hardware.