The answer to your question is yes, the WiSM blade will support your 1520 access point. You just have to be aware that the WLC is running the necessary version of softare. If you have a 1520 series like a 1522 you should not have problems because they are supported from 4.1 or 5.2 but if you are running later versions like 7.0 or 7.2 you should not have any issues.
You can see the release notes for more information about this and what access points are supported on which version:
http://www.cisco.com/en/US/docs/wireless/controller/release/notes/crn73.html
I hope this information helps you out.