If your library has only one card in it when Blurry Visionary’s ability resolves, it won’t make that card a modal double-faced card, but you will put it into your hand.

Once Blurry Visionary’s ability finishes resolving, you can’t change which face of the new modal double-faced card is the front face. It stays that way for the rest of the game.

If one of the top two cards of your library is already a double-faced card, put it into the new sleeve with either of its two faces facing outward. Only the outward face will be part of the new modal double-faced card, while the inward one ceases to exist for the rest of the game. For example, if the top two cards of your library are Brainstorm and the modal double-faced card Valki, God of Lies // Tibalt, Cosmic Imposter, you will end up with a modal double-faced card that has Brainstorm as one of its faces, and either Valki, God of Lies or Tibalt, Cosmic Imposter as the other face, with either face as the front (your choice).

And if that’s not complicated enough, similarly, if one of the top two cards of your library is a transforming double-faced card, you put it into the new sleeve with either face of the card facing outward, and it still becomes a modal double-faced card at the end. So, for example, if your top two cards are Brainstorm and the transforming double-faced card Delver of Secrets // Insectile Aberration, you will end up with a modal double-faced card in one of the following permutations: Brainstorm // Delver of Secrets, Brainstorm // Insectile Aberration, Delver of Secrets // Brainstorm, Insectile Aberration // Brainstorm. Note, however, that Insectile Aberration, like most back faces of transforming double-faced cards, has no mana cost, so it is normally not possible to play that card from your hand.

More complicated scenarios involving two different double-faced cards or multiple Blurry Visionary triggers affecting the same card over and over will be left as an exercise for the reader.

A modal double-faced card can’t be transformed or be put onto the battlefield transformed (even if one of its faces was formerly part of a transforming double-faced card). Ignore any instruction to transform a modal double-faced card or to put one onto the battlefield transformed.

To determine whether it is legal to play a modal double-faced card, consider only the characteristics of the face you’re playing and ignore the other face’s characteristics.

If an effect allows you to play a land or cast a spell from among a group of cards, you may play or cast a modal double-faced card with any face that fits the criteria of that effect.

If an effect allows you to put a card with particular characteristics onto the battlefield without instructing you to play or cast it, consider only the characteristics of a modal double-faced card’s front face to see if that card qualifies. If it does, it enters the battlefield with its front face up.

The mana value of a modal double-faced card is based on the characteristics of the face that’s being considered. On the stack or the battlefield, consider whichever face is up. In all other zones, consider only the front face. This is different from how the mana value of a transforming double-faced card is determined.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.