Hi,
Pin C.3 on a 14M2 is "Input Only", so it's strictly not an "I/O" pin and can't be used as an Output. But sorry, I don't know Blockly well enough to say if a "workaround" is needed to use it as an input.
In the Basic Compiler, an internal "Weak Pullup Resistor" can be used in some circumstances as a "Quasi (High Impedance) Output", but this isn't a feature one would expect a Blockly / beginner to use.
Cheers, Alan.