detect dark spots

Hello!
Help me plis. How detect dark spots? Dark spots of various shapes and sizes. There are many different pictures. Tried the usual threshold - does not work. Spots on various places of the picture.
Thank!

Help me plis. How detect dark spots? Dark spots of various shapes and sizes. There are many different pictures. Tried the usual threshold - does not work. Spots on various places of the picture.
Thank!