Where I can found Block start date-time and end date-time in SIU23 message. (The block time is the time which restrict the appointment creation in that time)
Note that the semantics of a HL7 message must be defined by the sending/consuming systems (HL7 mainly defines the syntax of the messages).
You are probably looking for the TQ1-7 (Start date/time) and TQ1-8 (End date/time) fields of the SIU message. But you should double check with the documentation of the sending system.