I’d recommend throwing the Uno in the bin and connecting the PH meter directly to the NodeMCU. But if you have a valid reason for not doing that then the other (less desirable, but more conventional) approach is to run the factory AT firmware on an ESP8266 based device. This is usually done with an ESP-01 board rather than a NodeMCU, but both are possible provided you can find the AT firmware to upload to the board.
More info here…
At the moment, your NodeMCU sketch is a total mess.
You are waiting for serial input from the Uno in your void loop and serial printing the characters one at a time as they arrive. The code attempts to assemble these into a string called I, which is then sent to Blynk.
You should NEVER have send data to Blynk in your void loop
Getting Blynk to co-exist with a function that listens for incoming serial data from another source such as the Uno is extremely difficult, and should be avoided in my opinion.
By far the best, and probably simplest, approach is to scrap the Uno and use the NodeMCU to read the PH sensor.
Pete.