What is the port of the blade SSD ？

I don’t have a simple answer as it depends on what your system came with!

• If you had bought your system with a HDD only you only have a SATA port (the PCI interface may not be populated on the logic board! The low end early 2013 system did not offer it

• If you bought a SSD model then you have a mSATA III blade SSD installed! The SATA cable would need to be installed iMac 21.5" (Late 2013-2017) SATA Cable as well as iMac Intel 21.5" and 27" (Late 2012-Early 2019) SSD Temperature Sensor and lastly, the mounting hardware to fit a 2.5” SATA SSD to the 3.5” HDD frame.
• If you bought a Fusion Drive system, you have both drives present

Give this a good read The Ultimate Guide to Apple’s Proprietary SSDs for deeper details.

OK, so adding or replacing the SATA HDD with 2.5” SSD would be the easiest direction and likely the cheaper. But that will require loosing the HDD and your stuff on it unless you’ve made a TimeMachine backup.

If you want to gain the most I would replace the custom Apple mSATA blade SSD with the OWC Aura Pro X2 unlike the Apple SSD which was mSATA the OWC SSD is a NVMe – PCIe 3.1 x4 drive (only able to use two lanes) which will give you much more performance as the boot drive! It will require you are running High Sierra or newer.