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I was browsing through a forum about pyramiding because i was interested and it looks like it's beginning to develope. Anyways I came upon a post that said you can put a peice of blackboard chalk inside the water and the chalk will dissolve. So does this mean the calcium blocks that i buy are nothing more than blackboard chalk?

No. According to the ingredients, cuttlebone is calcium carbonate while chalk for blackboards is gypsum (calcium sulfate).

Cuttlebone is very inexpensive as it is, why risk changing to chalk? I'm not sure what the difference in the compounds would do.

I made chalk in my chem lab last semester, we used just calcium carbonate and water, but I could see that some companies might add other ingredients to their chalk, pigments and the like.

Although chalk is non toxic, and would probably be safe, considering the box I have says that it is both non toxic and calcium carbonate, I would probably not risk it.

As far as I know, and I could very well be wrong, as I am not a chalk expert, Calcium sulfate is only used in chalks to make pastels, which have clay and oil also in them for use on paper.

We do consume Calcium sulfate, it is found in almost all baked goods (baking powder), as well as many pills, canned foods, it is a firming agent, so even if you did use chalk made from calcium sulfate (with no other added ingredients, of course) it would be as harmful to your turtles as baking powder.

Gypsum, which is hydrated calcium sulfate (CaSO4 * 2 H2O) is the chemical form found in plaster, cement, drywall etc.

Don't use calcium blocks either. An occasional piece of cuttlebone or dusted pellets (with Rep-Cal Calcium) should be fine.