I just did on my own.. Well what I needed was a needle, some rubbing alcohol, flat black spray paint, 400 wet or dry sandpaper and KwikGrip contact adhesive. All these things can be found from Home Fix.
1st I pried the dot (!) using the needle..I tried to turn the dot but to no avail..must do carefully or else end up scratching the paint on the body
Once the dot came off, I scraped all the red paint with the needle and alcohol..actually alcohol alone will do but I used the needle to get into the detail between the letters.
Then rub off the surface of the dot with the sandpaper lightly, just to key the surface so the paint will stick to it better..but I suggest don't do it on the Leica lettering as you want that part to be shiny later on, spray paint as per normal (don't rush, I did 2 layers of paint) then leave it to dry
After the paint dried, I carefully removed the black paint on the lettering with needle, but people on the internet used very fine emery board...
Remove the old glue at the back of the dot with alcohol, apply the Kwik Grip and you're done.
If your M8 is in immaculate condition, I would suggest that you get the real black dot from DAG, but his ordering form thingy is not working for so long and he won't reply emails lately. U might want to check on eBay also..but it is rare and 3x more expensive than DAG