Block vs Sheets
I prefer to buy my watercolor paper by the sheet. Actually packages of sheets 10, 50, 100 sheets to a package. I can then paint really large or trim the sheets down to the exact size I would like to use. It is very economical too.
But I do see the allure of blocks. Blocks of watercolor paper are excellent for travels and painting outdoors. The sheets are always the exact same size and ready to go.
If your watercolor painting style is rather wet, you are better off with sheets. Otherwise, I recommend slicing off one sheet from the block and mounting it on a board before painting.
Watercolor blocks usually come with around 20 sheet to a block when they're made up of 140 lb. paper. They are sealed around all four sides with a gum to keep water from getting to the other sheets. There is a small space usually at the top where there is no gum. That is where you slide in your palette knife and draw it all the way around the edges to remove the top sheet.
One thing I found with blocks is if you paint really wet, the sheet below also gets wet. After awhile, the gum either wears away or you get a little slit forming in the side and water seeps in. If the paper gets wet too many times, it begins to lose its sizing. If this happens, you'll notice an odd pattern around the edges of your painting. A weird texture appears in what should be a flat wash.
Blocks do have their benefits. Perfect for those who don't paint too wet. Great for traveling and painting outdoors. Another pro to the block, no watermarks or embossings.