Sam, not even sandblasting will reduce sharpness very much. Contrast, yes, sharpness not a lot. I say this after trial shots with a 135/4.8 Lustrar whose front cell's rear surface looked sandblasted and with a 210/5.6 Zircon whose front surface is just horrible. The Zircon shots even had good contrast.
Not sure what to suggest besides Rich's vaseline on a filter trick.
Someone used to sell a device called a Vignetta (sic) that was a diaphragm that attached to the front of a lens. Devices like this reduce coverage. Your black sharpie should have had much the same effect, perhaps you didn't darken enough of the front element. Maybe a Vignetta plus vaseline ...