[Casiopea]

Having to remember which verbs take -ed and which verbs take -t is not all that efficient for the language user. In Nature, as well as in other systems, the more regular the system, the more efficient the system. That strive for effeciency is the driving force behind irregular forms becoming regular (e.g., in language, BrE: learnt ~ learned. 'learnt' is not all that common these days in BrE. That's not to say British speakers are adopting American forms. No, not that. The reason has to do what's easier to remember. Verbs that end in both -ed and -t (e.g., burned, burnt; dreamed, dreamt) share a common characteristic with verbs that end in -ed (e.g., walked, lifted). Given that irregular system (i.e., some have both forms, some have one form) it's more efficient to divide the suffixes:

Use -ed for verbs (walked, burned, dreamed), and leave -t for participles (dreamt, burnt).

Please note, blest, according to the Oxford Dictionary of Current English,is considered poetical. That is, blessed is the common past form.