If that monster isn’t face-up on the field when this effect resolves, no card is searched.
If you control this card, and have a "Blackwing" monster in your Deck with less than 1300 ATK: When you Summon "Blackwing – Blizzard the Far North", both its effect and "Black Whirlwind" will trigger. Since they are both optional effects, you can order them in any way you like. If you order them as: Blizzard Chain Link 1, Whirlwind Chain Link 2, then your opponent can only respond to the newest chain link – Black Whirlwind's effect. This means that cards like "Royal Oppression" and "Pulling the Rug" cannot be activated at this time – Those cards cannot be chained to Black Whirlwind's effect.
The effect of "Black Whirlwind" starts a Chain. If an effect is chained, so that the Summoned 'Blackwing' monster is no longer face-up on the field when the effect of "Black Whirlwind" resolves, you cannot add a monster to your hand.
"Black Whirlwind" looks at the ATK of your newly-Summoned monster as it is on the field. If a Field Spell Card is increasing the monster's ATK, you can search your Deck for a 'Blackwing' monster that has less ATK than the increased ATK value.
If you control multiple copies of "Black Whirlwind", you can activate all of their effects. They form a single Chain, and you add 1 'Blackwing' to your hand as each one resolves.
If control of the Summoned 'Blackwing' monster shifts during or before resolution of "Black Whirlwind", you can still add a card to your hand because you can still see the ATK.