gutter residue

When retrieving my kid's ball from my gutter, I discovered about an inch deep black residue on the bottom of the gutter, I suppose from the roof tiles. Does this need to be removed/cleaned? It is not blocking the drainage as far as I can tell...
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An inch is a lot, and takes that much room away from the gutter's ability to move water. By all means clean it out - couldn't hurt!
A nice power wash should blast it right out.

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