The British Olympic champion stopped his Welsh opponent in the second round to make it four wins out of four.
The 24-year-old was in control of the bout right from the start and forced Darch to wobble on multiple occasions with a series of punches.
"Not too great", ESPN quotes Joshua as saying when asked about how he felt after the fight.
"A few things I was noticing in there when I was getting my jab going and my one-two, but you know how it is, the first round you are just warming into the fight.
"I was trying to capitalise on some of his weaknesses from early on, but as round two came along I started finding my rhythm. The next fight in March, hopefully you will see a bit of improvement."
Joshua's next fight is in Glasgow on March 1.