Some people do this here and frankly, the holes in the cinder blocks are not large enough to sustain plants well. The roots fill up the holes quickly and then there is no room for more growth, water etc. So consider using the blocks but plant behind them to cascade over. For an even cheaper solution, you can get wood from Walmart made to make raised beds with. Each piece costs about the same as two blocks and is 6' long. Just stack two on top of each other and screw them together and it will probably be a lot cheaper and easier for you to deal with in the end. No having to hide anything. My friends here also use pieces of logs for their raised beds and it is a nice, natural look at NO expense, usually. They will ask for wood on Freecycle or pick it up on the side of the road.
As for cascading plants, two I can think of now that would work for you are creeping phlox subulata and perennial candytuft.

Is your zone too cold for cascading rosemary? I have one plant that is really vigorous and has been in the ground for 3 years. It has been blooming (tiny blue flowers) for several months now. It blooms off and on over the summer, too.

Cameron

I was thinking along the same lines as GGG, I think the roots on most things would fill the blocks up quickly and watering would be a problem. On the other hand what about Sempervivums or creeping sedums. Lots of different ones to choose from, they don't need much soil or water and the form quite large clumps in rock walls so I don't see why they wouldn't work in your cinder blocks. The link below shows the many different types and colors you could choose from.

Annette

Here is a link that might be useful: Sempervivums

My ex-neighbor did this. They had a ring of cinder blocks around the base of a tree. They put Creeping Jenny in it and it did perfectly fine. Is your bed in full sun? If it is you might not want Creepin' Jenny, because it will fry.
You may try Creeping Thyme or a succulent. Cerastium tomentosum 'Snow-in-Summer' could work.
I have a row of cinder blocks along one little part of the fence-line where my neighbors river rocks are spilling over in my garden. I didn't want flowers in them, so I put those long, grey patio pavers (the cheap ones that are about 14in long and 7in wide or so). One paver fits perfectly on one block. I like it because you can sit on it and also line up pots along it.
Hope this helps you.
CMK

My choice would be cheddar pinks, like Bath's Pink or Fire Witch. They'll crawl onto a rock and grow, they should survive a concrete block.

Nell

You can use sempervivums, saponarias, thymes, sedums. Also look up alpine plantings - including thrifts, dianthus, etc. These are plants suited for little or no gravel at all and able to survive very harsh winter conditions. I think you will find that not only are they hardy but lovely as well.

Ianna