The first thing I know you are noticing is that 'BLKmage' is both a Monster card and an Attack Card.
'BLKmage' is a special Monster card that can be used in the Stack or as an Attacker but not both. Once this card is in play it must be one or the other. Although it's stats are quite low and Stuck value very high it is not a powerful card but it's versatility makes it a worthwhile addition to your deck.
'Space Shadow' is a simple monster but with a great ability. You may be using those Heart Cards up fast and with no way to collect anymore you only have the amount you put in the deck. This is where 'Space Shadow's' ability comes in. Retrieving them from the discard pile means you will not be running out.
These two cards will probably mix up your deck. You do not need to put as many Heart cards in your deck if you include 'Space Shadow' and you do not need to put as many Attacking cards in your deck if you include 'BLKmage'.