Black is not necessarily out of flavour, unless you're interested only in strictly honourable samurai. By splashing black, you could add a much less savoury aspect to the deck without abandoning the samurai theme at all.
If you don't like Mothrider Samurai, you might consider replacing it with Nezumi Ronin. Granted, the rat is easier to kill in combat, but when unblocked it's as dangerous as any other Samurai short of [card]Samurai Enforcers[/card], which costs
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Black doesn't offer anything else particularly worthwhile in the way of creatures, but some of its spells could give the deck a boost. Since samurai have pseudo-evasion, you can bet that people will avoid blocking them until they absolutely have to. Having two or three copies of [card]Dance of Shadows[/card] could ensure that when your opponent would actually want to block, all of your creatures have fear. Since you already have a few Otherworldly Journeys in the deck, Soulless Revival would be pretty good if you find your samurai are mixing it up with your opponents' creatures more often than you expected. If Reciprocate isn't doing the trick for you, Rend Flesh and/or Rend Spirit could be worthwhile.
