How to avoid Multiple definition of 'Blynk' error?
The Blynk "simple" header files are not ready to be included into multiple cpp files. They are meant to be included only in the main ino file. The reason is that the variable allocations are in headers file, which is bad practice for  C and C++. The h files should contain only declarations. The right way would have

extern BlynkWifi Blynk;


and the lines which are now in the h file should go in a cpp file. Then every cpp file that includes the .h will know about the variables but the variables would be allocated only once.

The fix is to change the library or to copy the BlynkSimpleEsp8266.h include to your project as for example MyBlynk.h and change the Blynk variable in MyBlynk.h to extern and add a MyBlynk.cpp file with variables allocation

#include "MyBlynk.h"

WiFiClient _blynkWifiClient;
BlynkArduinoClient _blynkTransport(_blynkWifiClient);
BlynkWifi Blynk(_blynkTransport);