Get the free version of Img2icns and open the program. Drag a Black Chrome png file (or any other picture you'd like to use) into the program window, choose "icns" as export option and choose a path where to put the .icns file. Next, right-click on Chrome in your applications folder and open "Get Info". You'll see a new window with a small Chrome icon in the upper left corner. Now locate your newly created .icns file and drag it into the "Get Info" window from Chrome. Drop it in the upper left corner, right above the original Chrome icon. A small plus sign should appear. Now when you drop the .icns file it'll replace the original icon. The change will appear the next time you open Chrome (application must be quit for the change to take effect). Every time Chrome gets updated you'll have to replace the original icon in the "Get Info" window by dragging the .icns file into the "Get Info" window, so you might want to keep the .icns file somewhere on your hard drive.