Solemn Offering should probably be in your main deck, rather than sideboard. I suspect in general you'll have issues against destruction-heavy Black decks and/or counter-heavy Blue decks: you're not doing much beside defending from attackers and gaining life. Your sideboard seems to be just more of the same old thing, rather than being set up to cover weaknesses you might encounter. There's not much point to swapping an apple out for an apple, when your fruit salad really needs a cantaloupe.
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I've updated it with some other cards I've got currently available, and raised it to a 60 card count per Standard/Extended requirements. A bit less life gain/drain, but hopefully more power and more answers. (I'm trying to keep this deck entirely with cards I currently own, but I'll keep my eyes out for the stuff you've recommended.)