Black scab after breast reduction, has something gone wrong? (Photo)

Cosmeeko, indeed the black area you see is a bit of skin that has not survived.  The terms necrotic, necrosis, dead are all appropriate, but don't be worried.
There are no signs of infection.  That black area is indeed dead and will eventually come off.  Likely the area will heal in on its own with skin growing from the healthy edges.
This will take about 3-4 weeks.  Your current care is fine.  If you get significant redness around it, or it's growing significantly get checked out, otherwise an appointment in 10-14 days should be fine.

These wounds appear stable.I would await the return of your plastic surgeon and do as recommended.Simple management is appropriate for now and delayed healing will likely follow.Patience should result in an acceptable outcome.

Those wounds will heal well, but will look worse before they look better. The area does not look infected and simple dry dressings to the area are sufficient for now. 

Unfortunately, delayed wound healing is relatively commonly seen after breast reduction surgery.  It is likely, that your areas of concern will look worse, before they look better. The "scabs"  tend to lift leaving an open wound (often with white or yellow drainage) that will then heal slowly over the course of a few weeks.Close follow up with your plastic surgeon will be very important; sometimes removal of a exposed suture and/or removal of any unhealthy tissue will expedite healing.  There may be different treatment modalities recommended; best to "stick with" your plastic surgeons recommendations. Generally, this will involve application of some type of sterile dressing ( in my practice I use a non-stick dressing covered by a sterile dressing once a wound is clean).You should have peace of mind that these types of wound healing problems generally go on to heal over the course of the next several weeks, often without long-term sequelae. Also important to concentrate on a healthy diet, including good protein source.  Sometimes (depending on the width of the open wound and the appearance of the scar in the longer term), scar revision surgery may be helpful down the line. Best wishes.
 

It looks like you have some skin loss.  Many times local wound care is all that is needed to get this to heal. Best to follow your surgeons instructions.