In interchange++, the margin you pay to an acquirer is always the same amount. This is regardless of the card type or how secure the transaction is. The total amount that you pay varies per transaction because of interchange and scheme fees but Interchange++ gives you the advantage of full transparency on the costs.
With blended pricing, you pay the same price per transaction per card regardless of the card and transaction type. This is a simpler way to calculate your cost but it does not consider all the cost variations.