We've played a few Elder Sign sessions at my local games store and we've worked on the assumption that all the investigators can combine their trophies in order to buy a blessing. Having bought UF and read the rules, it seems to me that the cost has to be paid by a single investigator physically travelling to the Chapel. Am I correct?

You are correct. There is no combining or sharing of items or trophies in Elder Sign (at least, not yet).

Thanks for the clarification.