You could just use one of the adjacent blades to install FreeNAS or similar and offer a full range of connectivity options (iSCSI, NFS, SMB etc. ) and other infrastructure or virt related services (even purchase an old blade for $1-200 to fill the role, many dirt cheap options out there). Thats assuming you go with 2.5", I just think it would be cheaper to buy a used 12 or > LFF server and use your stack of 3.5 inch drives. You would probably spend less than purchasing a storage blade and the 2.5 inch drives to populate it, plus gain the additional capabilities and expansion options vs. a pass through interface to slow spindles with limited capacity.
Which interconnect modules does your chassis have, also, does it have a full compliment of fans and power supplies?
I am assuming since you don't have a rack this may not be in a climate controlled environment, using spinners will generate a lot of heat in the chassis and it will start shutting things down if it detects an over temp condition. If you got it cheap because the vendor only provided a couple PS and minimum FANs, the storage blade or other components may not be able to draw enough power or cooling. A loaded out blade center can draw up to 14.4 kw (?from memory), most of mine have run from 6 -10kw on avg., my smallest config of 5 bl460c G8's, 2 VC-Flex-10 interconnects and 2 OA's draws 3 avg to a peak of 5kw. I have never used a storage blade so I cant provide specifics, just make sure your chassis has the required cooling and power for what you are attempting to configure.
Have you actually powered it up yet? Just thinking I have never seen anyone use the 110v option, just 220 or DC, anyway - just one more thing to check if you have not already.
I don't know of any solutions that would allow you to use 3.5" drives internal to the chassis, it sort of goes against the blade center "smaller and smushed together design principle.