Black Patch Under Cockpit

Date:         25 Mar 97 03:38:11 
Organization: Monmouth Internet
Followups:    1 2 3 4
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Can anyone tell me what the black patch under the cockpit window is?  I've
seen it mostly on older aircraft.  But, when those older aircraft get
repainted or change livery, the black patch disappears, although I have seen
it even on newer planes.  So what is its purpose?  Obviously its not necessary
or mandatory, so why did they bother?