It isnt much to go on, but if you see someone with, say, a large amount of Unglued Plains, Mountains, Swamps, and Islands for trade but no Forests, alarm bells should ring in your brain
A good reason for going mono-black is that black can cover almost all bases well (except artifact destruction)
2 Gigaplant 3 World Carrotweight Champion 2 Dust Knight 2 Lord Poison 3 X-Saber Fulhelmknight 3 Rose Lover 2 Thorn of Malice 2 Dark Core 1 Half Counter
This means you could put this mana into something like Urza's Ruinous Blast or Karn's Temporal Sundering at a pivotal moment
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