You have to use Blynk Widget Digital D9 to control the onboard LED. Must be a mapping problem in the Blynk or Wemos drivers. Here’s what I’ve discerned by trial-and-error on my Wemos D1 mini:
Wemos pin Blynk Widget Button ESP8266 pin
---------- ------------------- ------------
D0 D2 16
D1 D3 5
D2 D4 4
D3 D8 0
D4 D9 2 (Built-in LED)
D5 D5 14
D6 D6 12
D7 D7 13
D8 D10 15