How can I get a variable from espruino web ide in blockly code?(MDBT42Q)

I think it's enough just to define a variable called ir_adc in Blockly (assuming you already have your code uploaded to the MDBT42Q). Either that or pull in the "A JavaScript "+"Expression" and just type in ir_adc

However it sounds like you want to make a block for it in the IDE code itself? If so it's as easy as just making the block return the next ir_adc - for instance you could just use this: https://github.com/espruino/EspruinoWebIDE/blob/gh-pages/blockly/blockly_ble.js#L376-L385