The distinction between the words blessèd (/ˈblɛsəd/) and blessed (/blɛst/) (see Grammarbook) appears to be wearing thin in modern language, possibly due to reduced accent usage and its resultant lack of different pronunciation between the two terms.
Wiktionary refers to the word as
And most damningly of all as an
Alternative form of blessed (verb)
While I understand that little usage of terms such as blessèd and cursèd cause them to get blurred with their verb counterparts, some adjectives like naked, talented, sacred are still used, always written without the accent. In some legal and political circles†, learnèd is still used for specific traditional purposes but it is now starting to fade.
What is the history behind the distinction in pronunciation?
The reason why some of these words are pronounced with /əd/ is explained by the answers to this question: "Why pronunciation of “Crooked” is “Crook-ked”?"
David G said:
If the normal form of the word (crook, twist) ends in t or d, the -ed is enunciated (such as wanted or bedded).
This explains the pronunciation of talented, but not that of naked, sacred, or blessed. The pronunciation of naked is however explained by coleopterist's answer to this question: Where does “wicked” get its /ɪd/ from?
According to A Critical Pronouncing Dictionary and Expositor of the English Language, 1839,
The adjectives naked, wicked, picked (pointed), booked, crooked, forked, tusked, tressed, and wretched, are not derived from verbs, and are therefore pronounced in two syllables. The same may be observed of scabbed, crabbed, chubbed, stubbed, shagged, snagged, ragged, scrubbed, dogged, rugged, scragged, hawked, jagged; to which we may add, the solemn pronunciation of stiff-necked; and these when formed into nouns with the addition of ness, preserved the ed in a distinct syllable, as wickedness, scabbedness, raggedness, &c.
However, the answers to "Pronunciation of "Blessed"" don't describe the historical origin of the different pronunciations of this word as adjective and verb form.
What is the history behind writing these words with accents?
How common was it in the past to write these words with grave accents, and in what contexts did people do this? There has already been a question about the use of this accent in Shakespeare's work: "Are there any pairs of words like “beloved”/“belovèd”, “learned”/“learnèd” that maintain a semantic difference to the present day?" But it's not clear from that if Shakespeare's original manuscripts use it, or if it something that's been added to modern editions to help readers find out the meter.
Why have these words generally lost their accents?
Has this then lead on to them losing their differing pronunciations?
Finally, should one use a grave accent (è) here or should one just let context dictate pronunciation?
†Interestingly, Hansard don't include accents at all, even when one looks back into the 18th and 19th century records. This may, however, be a result of typewriting and modernising of the records.