Yeah, go ahead and plant in the blocks, but go with small annuals, like pansies, strawberries, marigolds, etc. Perenials are not going to do well there, as a rule. (and we know about exceptions...)
In this case, paint might not be a bad idea, particularly on the inside of the block to keep the moisture in the soil instead of leeching out the brick. Use latex, or use a concrete sealer.
Good luck, and get that green thumb looked at.
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