The way that digital datastreams has been implemented in Blynk IoT makes no sense at all as far as I’m concerned, and I’d suggest that you stay clear of them until they are re-implemented in a way that matches how they worked in Blynk Legacy.
Have a read of this…
But, even when (if) digital pins are implemented properly, I’d still recommend using virtual pins as they give much more flexibility and also make it much easier to swap your sketch between different hardware types if needed.