The SSD blade drive is a very thin SSD that looks a lot like a memory module. It connects to a PCie slot instead of an HD sata port.
According to EveryMacs overview:
For storage upgrades, if the 21.5-Inch "Late 2012" iMac models were not configured with an SSD or a "Fusion Drive" at the time of purchase, there is no way to add a PCIe-based "blade" SSD later as the needed connector is not present. However, the 27-Inch "Late 2012" and all "Late 2013" iMac models do have this connector on the motherboard even without a pre-installed SSD, so it is possible to add a PCIe-based "blade" SSD after the purchase of the system.
http://www.everymac.com/systems/apple/imac/imac-aluminum-tapered-edge-faq/differ ences-between-imac-aluminum-late-2012-late-2013-models.html
One of these should work:
http://www.newegg.com/Product/ProductList.aspx?Submit=Property&N=100008120%20600 038516%20600038517%20600038515%20600489717%20600171532%20600171533%20600488413&I sNodeId=1&name=M.2
With that said you will have to completely disassemble the iMac to install an SSD blade in it. That means not only removing screws, but also glue, and other securing mechanisms, not easily removed.