The Blank White Card is... well, you see, it's a card, probably white, and I'll bet you money that it's blank. In more useful terms, it's the basic material for 1000 Blank White Cards. In other jargon, it's the primary resource, tabula rasa, infinite canvas, or El Naipe Blanco y Virgen. Once a player draws a blank card, they may immediately start drawing whatever on it (barring any restrictions from other cards, of course).
The current standard is 3" x 5" unlined index cards bisected widthwise. However, variants exist for pragmatic reasons: lined cards provide an easily identifiable back, full cards provide more space, A7 cards can be made extremely easily out of A4 paper, etc. The cards may not even need to be white, or even totally blank. If you want to go really crazy, maybe not even cards in the strict sense of the word.
It is normal convention (Or to put it another way, the point of the game) to draw upon these blank white cards any picture, title, effect or other thing they wish. It is not unheard of to modify the cards by taping things to them or mutilating them.
A blank white card can usually be considered a far better draw than an already drawn-on card, as it can be manipulated to solve any problem the player faces, be it a particular card, a group of cards, or an opponent's refusal to shut up.
There are three basic "series" of cards in any given deck, classified depending on the situation they were created in.
First series: Fresh Ideas
These are cards created for not really any reason other than that the creator has a cool idea for a card (which we all get from time to time). These tend to be of fairly high quality, and generally occupy the top 3 levels of Professor Ickbweck's Sliding Scale of Artistic/Notational Tendencies, as the effect often takes a back seat to the art.
Second series: Cards of the Now
These cards are the ones that occupy the rest of the layers of Professor Ickbweck's Sliding Scale of Artistic/Notational Tendencies. They're created when somebody has a problem they need to solve. Often lack titles for some reason, and are usually of slightly lower quality than First Series cards. A great deal of these are Card Manipulation Cards, because quite often players will want to discard boring cards from their hand or remove irritating cards from play. These are created to get the player out of whatever trouble they're in at the moment.
Third series: Blankfiller
These cards tend to be the lowest quality of all, although not always (Sometimes they can be great and popular) . These are created when someone:
A. Has a lot of blanks in their hand.
B. Lacks inspiration.
These can fit anywhere on the sliding scale. Sometimes someone just slaps a point value on an otherwise blank card, and sometimes they will just start doodling hoping that inspiration will strike.
While definitely not recommended, it is possible to play a blank right out of the box. This would be extremely similar to playing a Useless Card, but the main distinction is that while Useless cards have some art and a title, playing a card that affects blanks would either be required to affect cards which lack a certain trait/combination of traits or to affect blanks directly.
It is unclear whether or not blanks can be legally drawn upon after being played, which is something you and your rare 1k friends will have to decide upon.
Ideas for Blank-Playing
(Bear in mind, most of these will require at least 2 blanks to pull off.)
Play a blank card facedown (it technically can't be facedown unless you and your aforementioned rare 1k friends use lined index cards, in which case, you should play these 2 cards in a short time span of one another.) to confuse opponents, then use the other blank to set a new winning condition (Whoever plays the most Blank cards wins the game,) and finally let the realization hit your opponents when they look at your supposedly facedown card. (see also: Fifteen Turn Double Reverse Snapback with Sprinkles)
Create a blank-eater card which grants a large amount of points to any player (not just yourself, or you may become a target for actions) who removes a blank card from the game. Each player now has a choice between keeping the all-powerful in their hand, or trashing them for points.
Play multiple blanks in your play area, but keep at least one of them to create an especially devious card. (If there are any Blank Cards in your play area, you may create cards from them as if you were playing them on yourself for the first time.) Let your opponents watch in horror as you amass the power of multiple blank cards in the span of one turn.