What does it mean for a card to be "Blocked"
First and foremost, know that most tarot readers consider any card which presents itself on it's side, to be a blocked card. When the card is in it's "normal" view, it's Upright and when it is "upside down", it's considered Reversed.
These labels are simple and really only suggest that the energy coming from the card holds slightly different meanings in each position. Upright is generally considered to be the most positive positive, reversed to be the negative or opposite and "blocked" often means that the energy that the card is talking about, is either not available to the querrent or that it is impeding the querrent from their desires. Hence the term "blocked".
To give you an example, let's talk for a moment about the Tower card from the Major Arcana.
The Tower Card
The Tower sits between the Devil and the Star in position number 16 of the Major Arcana. As we finally gain an understanding that the Devil is simply a deeper side of ourselves and nothing to fear, we are truly put to the test with the Tower card, the card of absolute change, a scary thing for most people regardless of how large the change might be.
The Tower has a very specific purpose, for the change it creates becomes the catalyst that will bring out the Star card, one of true balance. Though the only way you'll ever achieve that balance is to jump.
When the Tower card comes up in a any reading, that is it's main message - Lead and the Net will Appear....
It's truly a card of faith that asks us to trust us when it says that change is coming and it will no good to hold onto the past which is metaphorically represented by the Tower in the image itself. It's up in flames and surrounded by a troubled ocean. There's no food or clean water anywhere to be found and the only light comes from the lightening which spontaneously struck the tower and started the fire.
Now, all of this that has been said, is true of the Tower card in it's upright position.
Once reversed, the card takes on a whole new meaning that plays upon the same themes as the upright position. In it's reversed format, the Tower card is still all about change, though instead of the message being "jump" it's more like "Hold on like your life depends on it!"
In it's upright position, the card is rather soothing, suggesting that although a change is coming that you cannot stop, it means good things for you and those around you. The Tower in reverse suggests that it's going to feel like there are great changes taking place and maybe even as though there is a bunch of chaos in the air, but really there is not and this is simply a time when you want to wait it out.
The Tower card Blocked
Now when it comes to cards that are "blocked", there are two ways the card might lie - to the left or to the right.
There are many different ways to interpret this. The most common is simply that all side cards hold the same meaning. Though many readers, myself included, study every tiny detail about the ways in which cards communicate to them and that means that the way the card appears to us is extremely important. Even minor details count for the thorough reader.
It's not much more complicated though, as the cards want nothing more than for us to be able to easily communicate with the universe and that wouldn't be possible if the cards were so hard to interpret.
Blocked to the Left - when a card is laying on it's left side, that suggests that the querrent or subject of the reading is experiencing blocked energy that is leaning towards a reversed position.
Blocked to the Right - when a card is laying on it's right side, that suggests that the querrent or subject of the reading is experiencing blocked energy that is leaning towards an upright position.
Hopefully from those summarize explanations, you can see why it would make a difference. If the querrent's card is heading towards a reversed meaning, often times that means the reader can help by letting them know what ways they might bring that card into an upright position instead.
As an example, the other day I was doing a reading for myself about a certain job I'm working on with a team of others, and the Tower card appeared to me as an answer about what would be best for me to do.
The card appeared to be blocked to the left, which presented an interesting image. With the Tower on that side, the woman falls blindly off of the tower instead of jumping and the man, who did jump off the other side, ends up landing on the side of the burning tower, likely injured by the look of his impact.
The meaning of the card in this position is much the same. Because it's to the left, the energy is impeded but still heading into a card of reversed position, which tells me that while I need to hold on while the chaos erupts, it's not likely to be pretty whether I do that or jump off.
Had the card been to the right, it might have told me instead that there was a chance to maintain a grip on reality while everything went topsy turvy, as the woman lands on the side of the tower in a different way than her counterpart would. She lands on her hands and knee's, able to see and hold on as the Tower comes upright again, slowly letting her enter the water as the time nears.
As with all Tarot subjects, the position of the card is subjective and based on the experience and comfort level of the reader themselves.
Some might ignore blocked cards and turn them either fully upright or reversed, while others very much appreciate another way to clearly interpret the cards messages.
It's up to you in the end, the reader and interpreter, to decide what the cards are telling you. It's my hope that some of the knowledge I've left here might help you in that quest, and I'm sure that it will so long as you remember that like the future, the meaning is never cemented in stone.