Using the Clone Stamp Tool
With Photoshop's Clone Stamp Tool, you essentially choose an area that is without blemish to copy over the place that has the imperfection. Below, the Clone Stamp Tool is selected, and the box to the right allow us to choose the diameter and hardness of the tool. The size of the tool will be determined by the size of the blemish.
This tool has a myriad of uses, but when we are trying to remove skin blemishes, we want to use the blend mode labelled Lighten. Below is a picture of the drop down menu. By choosing Lighten, the Clone Stamp Tool will only change and edit the blemish, which is darker than the rest of the skin. In this way, the lighter pixels will remain the same.
The next step is to choose the area to copy from. Using the tool, hold down the Option/Alt key and the tool will turn into an icon that resembles the crosshairs of a sight. Click on the chosen area that is clean, and the clone tool will use that as its template to copy over.
After that, removing the blemish is simply a matter of positioning the Clone Stamp Tool over the blemish and clicking. Instantly, the imperfection will be obliterated and replaced by the clean sample you selected.