Dark or blackish in color; close to black but not completely black. The suffix -ppoi implies 'ish' or 'like'.
その猫は黒っぽい毛並みをしている。
That cat has a blackish coat.
黒っぽい色の服は汚れが目立ちにくい。
Clothes that are blackish in color don't show dirt easily.
写真が暗くて黒っぽく写っている。
The photo is dark and looks blackish.