Blotched SSD upgrade: Why is my display black?

If your HD was working and you are swapping it out to gain the speed of the SSD you may want to consider putting it back and then add the SSD as a second drive following this IFIXIT guide: Installing iMac Intel 21.5" EMC 2428 Dual Hard Drive Kit. Once installed you can prep it up leveraging the internal HD and then install a fresh copy of OS on it and migrate your apps to the SSD. Once done you can then go into the system prefs and select it as your start up drive. Now with a dual drive you gain the speed of the SSD but also hold onto the deeper storage of the traditional HD. In this case the second drive does not need to be connected to SMC services. This is what I recommend as the ideal setup!

The issue with your internal display sounds like you forgot one of the cable connections. Review this IFIXIT guide to see what was forgotten or mis-connected: iMac Intel 21.5" EMC 2428 Display Replacement. To help narrow down what is disconnected try using a flashlight shining it into the glass at a sharp angle, you should see a faint image of your desktop icons if the backlight connection was forgotten. As you have an external display working we already know the GPU is OK, so that leaves the data cable and the thermal sensor cables.