Yes, but only up to a point.
The final tweaks, bells, whistles and flashing lights will be added to the custom version of the app as part of the customisation stage - provided that is possible.
So, with Blynk, the process should be to prototype your product to the point where you have the majority of the functionality you need, then to open a dialogue with the Blynk guys about what’s missing.
If you run into issues during the prototyping process then start talking to the Blynk guys earlier, so that you don’t waste time and effort.
In fact, as Blynk 2.0 is rumoured to be on the horizon, it would probably be best to run any ideas past the Blynk guys at a fairly early stage in the process so that they can steer you in the right direction. You may also find that they have some additional ‘goodies’ that you can use, which aren’t available in the free version of the app, and they may have some pre-built code that they can share with you for certain pieces of functionality.