I'Black Knot is a relatively common fungus (Apiosporina morbosa) that tends to affect the distal (outer) limbs of Cherry and Plum (and Peach).  As you can tell from the photos , it makes large black growths that are hard and dry to the touch (but harmless to you).  These growths will eventually cut off transport of water and nutrients to the ends of the branches and cause death past the growth.  It rarely kills a tree but generally makes a tree so unattractive that most want the tree removed.

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 This fungus tends to show up when the trees are in low light situations and  the tree is not at optimal health due to lack of sunlight. You can cut it out and that will help but if you don't deal with the lack of sunlight , it will just come back.  Giving the tree optimal sunlight can usually knock back more intensive outbreaks. 

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 Occasionally and unfortunately some trees are just genetically predisposed to getting this disease, especially in older trees  even if they are otherwise healthy (full sun). Fertilizing and crown reduction can help but there is no guarantee.   

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