The 'èd' verb ending in 19th-century and earlier poems indicates that you are supposed to pronounce the ending -ed as a separate syllable.
This is not the spelling Blake used when he originally wrote this poem. He did not put the accent on leaped and shouted.
So why do editors add the accent today?
Today, in English, there are two ways of forming past tenses of regular verbs. If the verb ends with a /t/ or /d/, we add an extra syllable, as in printed. Otherwise, we either add the consonant /t/ or /d/, as in laughed, grinned or echoed.
When Blake wrote, the convention in poetry was to pronounce all past tenses spelled with 'ed' with an extra syllable, even though these endings weren't pronounced in normal speech. If the poet didn't want an extra syllable, they would replace the 'e' with an apostrophe, as in cover'd, leap'd, and laugh'd.
Today, since people don't understand this convention, editors insert an accented 'è' in past tenses when the poet wanted you to pronounce an extra syllable, like
leapèd, shoutèd, and echoèd in Blake's poem.
How did this convention develop? In late Middle English, most verbs formed past tenses by adding an extra syllable. When Shakespeare wrote, in Early Modern English, the extra syllable was in the process of disappearing, and most regular verbs had two pronunciations in widespread use. Shakespeare and his contemporaries generally distinguished between these two pronunciations using spelling (although they didn't use accents and often didn't use apostrophes). Over the next two centuries, poets took advantage of the freedom to use these two different pronunciations so as to make their poems rhyme and scan, even though people had long stopped pronouncing the extra syllable in ordinary language (except for words like printed, where we still pronounce it today).
How should these be pronounced? If you look at the poems of Shakespeare and his contemporaries, they rhymed "-ed" past tense endings with words ending in /ɛd/, like bed and dead. Two hundred years later, Blake and Keats were still using those rhymes. Was this how this ending was pronounced in everyday English? If you look at Walker's Critical Pronouncing Dictionary from 1824, past tenses like blessed, learned, repeatedly, unrepented, are pronounced with /ɛd/, even though words ending with "-ed" that aren't past tenses, like naked, ragged, wicked, are pronounced with /ɪd/. So Blake and Keats pronounced the "-èd" ending the same way that (at least some) people pronounced past tenses of regular verbs ending with -ted or -ded.
So when you read poetry with -èd endings, how should you pronounce them? That's up to you; I can see arguments both for pronouncing it as /ɛd/, and for pronouncing it the same way you pronounce the past tenses of regular verbs like created.