Hello Detlef, 
 
I never saw a sumatra blade that was black due to forging. 
 
Occasionally I have seen recent mandau blades that where black forged, probably chinese machine made blades. But never on real indonesian blades. 
 
How is the black near the handle ? 
My guess is leaning towards black paint. 
If it was mine I would probably clean it... 
 
Best regards, 
Willem
		 
		
		
		
		
		
		
		
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