They were not so much designed, as were the prologues of the classical theatre, to enlighten the spectators touching the subject of the forthcoming play; but were rather intended to bespeak favour for the dramatist, and to deprecate adverse opinion.
Originally, indeed, the prologue-speaker was either the author himself in person, or his representative.

In his prologue to his farce of "The Deuce is in Him," George Colman, after a lively fashion, points out the distinction between the classical and the British forms of prefatory address: What does it mean?
What can it be?
A little patience--and you'll see.
Behold, to keep your minds uncertain, Between the scene and you this curtain! So writers hide their plots, no doubt, To please the more when all comes out! Of old the Prologue told the story, And laid the whole affair before ye; Came forth in simple phrase to say: "'Fore the beginning of the play I, hapless Polydore, was found By fishermen, or others, drowned! Or--I, a gentleman, did wed The lady I would never bed, Great Agamemnon's royal daughter, Who's coming hither to draw water." Thus gave at once the bards of Greece The cream and marrow of the piece; Asking no trouble of your own To skim the milk or crack the bone.
The poets now take different ways, "E'en let them find it out for Bayes!" The prologue-speaker of the Elizabethan stage entered after the trumpets had sounded thrice, attired in a long cloak of black cloth or velvet, occasionally assuming a wreath or garland of bays, emblematic of authorship.

In the "Accounts of the Revels in 1573-74," a charge is made for "bays for the prologgs." Long after the cloak had been discarded it was still usual for the prologue-speaker to appear dressed in black.

Robert Lloyd, in his "Familiar Epistle to George Colman," 1761, writes: With decent sables on his back (Your 'prologuisers' all wear black) The prologue comes; and, if it's mine It's very good and very fine.
If not--I take a pinch of snuff, And wonder where you got such stuff.
Upon this subject, Mr.Payne Collier notes a stage direction in the Induction to Heywood's "Four 'Prentices of London," 1615: "Enter three, in black cloaks, at the doors." Each of them advancing to speak the prologue, the first exclaims--"What mean you, my masters, to appear thus before your times?
Do you not know that I am the prologue?
Do you not see this long black velvet cloak upon my back?
Have you not sounded thrice ?" So also, in the Induction to Ben Jonson's "Cynthia's Revels," two of the children of the chapel contend for the privilege of speaking the prologue, one of them maintaining his claim by pleading "possession of the cloak." The custom of regarding the "prologuiser" as the author or his representative, seems gradually to have been departed from, and prologues came to be delivered by one of the chief actors in the play, in the character he was about to undertake, or in some other assumed for the occasion.

A certain solemnity of tone, however, was usually preserved in the prologue to tragedy--the goodwill and merciful consideration of the audience being still entreated for the author and his work, although considerable licence was permitted to the comedy prologue.