This is what he wrote:
Please change barcode so it is 100% black ink (K) on a white background. (A barcode can look black in the proof, but can be built as 5% Cyan, 5% Magenta, 5% Yellow, 85% K for black. The barcode needs to be built as 100% K black)
Does this mean that I have to set the CMYK value in Photoshop to 0,0,0,100?