The stonecutter blade has some weird shading properties that make it darker than it should be.

If you place the stonecutter facing north/south and put solid blocks on its sides the blade displays the correct shading.

Also, if you place a solid block only at its east side, the blade will become even darker.

Notes: This probably occurs because of the model UV only applying the blade texture to one face. This is similar to the bamboo, which was tweaked with a brighter texture. The same fix wouldn't work in this situation due to the varying shading.