I get lights connected to my Hue Bridge (v1) unreachable pretty frequently, though mostly GE Links which are known to be flaky. Sometimes a genuine Hue bulb.
“Unreachable” means that the Hue Bridge isn’t getting expected responses from the bulb over ZigBee Light-Link (ZLL)… a variation of ZHA (ZigBee HA) that SmartThing uses. They are on the same frequency 2.4ghz as WiFi, but not likely on the same “channel”. So … interference is possible.
Unplugging the Hue Bridge for at least 15 minutes while leaving all the bulbs powered on will force them to panic and then rebuild their mesh when the Bridge is plugged back in.
What does this have to do with Blue Iris? Nothing, really. Blue Iris should not be causing any issues, except the very slight chance that the “extra” WiFi 2.4ghz traffic is interfering with the ZLL mesh. That’s only slightly possible. Moving your WiFi router away from your Hue Bridge can help.