"My question is could the evil of the blade be 'cleansed' away? If yes why and how? If no why?"
Would it be interesting for the game/story for it to be possible? If so yes, if no then no. If you want it to be possible, explain the enchantment as being, for example, arising from a spirit bound into the blade, or anchored in the black opal or the mithril veins. A spirit can be redeemed, exorcised, freed, or destroyed, even if the blade can't be. The spirit angle gives more plot opportunities; does it have personality? Was it evil when it was bound in, or twisted to make the blade? Did it have goals? Is it capable of growth and change? Could it be redeemed through life lessons, by achieving objectives it long ago gave up as impossible, by exposure to holy forces, by being forgiven by a deity or an entity long forgotten? These things could be easy or they could be so difficult it takes the entire span of the campaign.
Materials-wise, black opal and mithril can be worked or destroyed. Perhaps with tremendous heat and force the mithril can be worked out leaving the laen as clear and pure as water. (Although, thematically, mithril tends to be good, so that's a bit odd.) Conversely, it could then be suffused with purified metal or other substances filling in those veins, and then polished back down to the mirror-like surface of the laen. The stone of course can be replaced.