Firstly its high sophisticated algorithm & yet under evolution progress world widely, for that they call it (blind) Cuz lack of control due to probable conflicting decision between source/target RRM entities(potential ping/pong).
2ndly the UE use the compressed mode just for same RAT, & so Inter Rat HO need multi-mode-device UE,
#Basically to perform such HO the UE need to switch to target technology (RX different tuning), & this make interruption time to measure the diff (radio, access tech, diff controller).
#the RAT/source decide the HO to UE & after that UE make the measurement on target RAT ?????,but UE make the interruption time used to (switch on & measure)on target RAT, by individually compression on source (UMTS band) to obtain such interruption space for target technology. So it’s not the compression mode under discussion.
But may there UE with separated RX processor & control the diff path simultaneously(I read about it but not much).