From Part-FCL Definitions:
"Flight time under Instrument Flight Rules" (IFR) means all flight time during which the aircraft is being operated under the Instrument Flight Rules.
"Flight time":
for aeroplanes, touring motor gliders and powered-lift, it means the total time from the moment an aircraft first moves for the purpose of taking off until the moment it finally comes to rest at the end of the flight;
Therefore, from those two definitions (and assuming the entire flight is operated under IFR), then it should be block time that is logged.
Not to be confused with Instrument Flight Time, which I believe is only used under EASA for the purposes of granting a licence or rating (e.g. logged as part of an IR course).
"Instrument flight time" means the time during which a pilot is controlling an aircraft in flight solely by reference to instruments.
HTH