Can black shadows harm someone for no reason?

It depends on what you are dealing with!
If it is a territorial spirit/ghost it should not bother you unless you bother it. However if you are dealing with a demon than it may most certainly attack you for no reason.

How to tell if it is a demon or spirit?
-The shadow of a spirit is faint whereas in case of demon it is darker.
-If it is a spirit it will be confined only to a particular place (eg for kitchen/bed room etc) whereas if it is a demon it may appear anywhere.
-The shadows of demons are aggressive, for example they may try to approach you but the shadow of a spirit is usually stationery.

Hope this helps..
P.S: If you want to remove a territorial spirit discover its thing or place of attachment. It will go away. If its a demon.. Contact a pro..