Blending Images into each other at Image Magic.
Image Magick
Place the darker image last. (on top)

Upload your first image, Than choose Composite and upload the Grayscale Image.
Click Here For Grayscale

Next choose CopyOpacity from the Composite Drop Down, then click the Composite button.

That puts the Grayscale on top of your first image.

Now choose Composite again, and upload your Background (second) image.

This time you choose DstOver from the Drop Down menue, and click Composite again.

To finish, you next go to Enhance and click Normalize. Now Download your image.