This logic is a bit confusing. I would prefer it as

    if (!adjacentBlocks) {
        adjactentBlock = Block.DIRT;
    } else if (adjactentBlocks != blocks) {
        adjactentBlock = ...neighbour
    } else {
        adjactentBlock = ...voxelated
    }

You could even put it in a function if you wanted (just local to simpleNewMesher).