Sorry but the picture quality is too bad to see what's going on, but the wound does not look reopened.
If the black substance is a scab, it's ok for the 7th day. However, if the skin itself is turning blue or black, your vet needs to have a look again.
The red edge of the incision looks slightly inflamed, which is no reason to panic. If the red spreads out, please contact your vet again. Keep in mind that fresh, healing skin looks reddish pink as well.
The removal of the E-collar depends on how much damage your dog can do to himself. If the stitches need to be removed, the collar should only come off after that is done. If the stitches are resorbed by the body, you can remove the collar, but keep an eye on your dog. If he licks or bites at the wound, you must put the collar back on.
As a general note: I'm surprised the vet didn't shave away more fur around the incision, but the picture could be misleading.