ERJ 170/175 First Officer reported the flight crew unknowingly transported 5 boxes of undocumented HAZMAT but its transportation did not exceed in any limitations.

Narrative

On my postflight inspection at ZZZ; I was notified by ramp personnel that there were 5 boxes of donated blood in the aft cargo hold; of which they were not aware of from any information and manifests available to them. These boxes weighed 185 lb. total across the five of them. Originally; I thought it was 185 lb. for each of the 5 boxes. This was later corrected by the Ramp Lead who conferred with myself and the Captain. The boxes were supposed to be offloaded in ZZZ1 from the inbound flight of that aircraft. A crew swap occurred and we flew it to ZZZ. After landing in ZZZ and being made aware to this issue; we inspected our takeoff data print out from software and observed that there was no listed cargo at all; totaling 0 lb.; in the aft cargo. We contacted relevant Maintenance Operations managers due to the HAZMAT nature of the boxes. We then received word to fly them back to ZZZ1 and received a manual paper report from the ramp staff in ZZZ to enter all the bags they loaded plus these five boxes. In ZZZ1 I ensured these boxes were removed upon arrival at the gate. It is worth noting that this did not result in any exceedances of any limitations. Cause: Likely improper procedures taken by the Ramp when handling the arrival of this aircraft from ZZZ2 to verify the aft cargo hold was checked for cargo. I also did not inspect the interior of the cargo hold on my preflight inspection; nor do I believe it is our procedure to do so. For unreliable passenger counts; we can conduct a manual count with our flight attendants. In this situation however; there are no existing fail-safes to ensure that the data given to us by the Ramp for report numbers is accurate. Suggestions: Possibly retraining of ramp personnel and possibly some system to verify bag and cargo counts. We routinely have to verify passenger counts when they differ greatly from planned values. But we have no such mechanism for cargo counts; partly because it is time consuming and complicated to recount all cargo and bags. For this reason; we absolutely trust that the values we receive for bag and cargo counts are accurate. For us pilots to verify the integrity of these values is also difficult. Ultimately it will probably fall on ramp personnel to ensure competence.