I am looking for how to export or save data from ethereum blocks in a json file using quickblocks.
The C++ code example on the referenced page in the above comments shows how to get a single block.
CBlock block; getBlock(block, 3500000); cout << block << "\n"
Simply surround that code with
for (int i = 0 ; i < getLatestBlock() ; i++) and the entire blockchain will get printed to the screen. Redirect into a file, and you have what you want--but I warn you: it will take up an astronomical amount of disc space.