Hi,
The idea is to hold the component with the blue tack then solder the leads on the OTHER side if the board.
Now what's wrong with the time approved piece of foam "rubber" and elastic bands? Leaves no residue at all, just populate, add foam and fasten with the elastic bands, turn upside down, bend-cut-and-solder the entire board in one go.
Or, if you prefer mounting them one at a time, get half an inch of silicone rubber tubing wide enough to fit the tip of your middle finger and use that to hold down (or rather up) the component while soldering.