The Mysteries of Udolpho

In Chapter 6 (Volume 1), Catherine gushes about The Mysteries of Udolpho by Ann Ward Radcliffe. The text of the book can be found online here. I did a quick search for references that Catherine made to the black veil, and I believe this section is it:

Passing the light hastily over several other pictures, she came to one concealed by a veil of black silk. The singularity of the circumstance struck her, and she stopped before it, wishing to remove the veil, and examine what could thus carefully be concealed, but somewhat wanting courage. 'Holy Virgin! what can this mean?' exclaimed Annette. 'This is surely the picture they told me of at Venice.'

'What picture?' said Emily. 'Why a picture--a picture,' replied Annette, hesitatingly--'but I never could make out exactly what it was about, either.'

'Remove the veil, Annette.'

'What! I, ma'amselle!--I! not for the world!' Emily, turning round, saw Annette's countenance grow pale. 'And pray, what have you heard of this picture, to terrify you so, my good girl?' said she. 'Nothing, ma'amselle: I have heard nothing, only let us find our way out.'

'Certainly: but I wish first to examine the picture; take the light, Annette, while I lift the veil.' Annette took the light, and immediately walked away with it, disregarding Emily's call to stay, who, not choosing to be left alone in the dark chamber, at length followed her. 'What is the reason of this, Annette?' said Emily,
when she overtook her, 'what have you heard concerning that picture, which makes you so unwilling to stay when I bid you?'

'I don't know what is the reason, ma'amselle, replied Annette, 'nor any thing about the picture, only I have heard there is something very dreadful belonging to it--and that it has been covered up in black EVER SINCE--and that nobody has looked at it for a great many years--and it somehow has to do with the owner of this castle before Signor Montoni came to the possession of it--and'---

'Well, Annette,' said Emily, smiling, 'I perceive it is as you say--that you know nothing about the picture.'

'No, nothing, indeed, ma'amselle, for they made me promise never to tell:--but'--

'Well,' rejoined Emily, who observed that she was struggling between her inclination to reveal a secret, and her apprehension for the consequence, 'I will enquire no further'---

'No, pray, ma'am, do not.'

'Lest you should tell all,' interrupted Emily.

Whatever could be behind the veil? I won't tell here, but maybe in the comments...

This brought to her recollection the veiled picture, which had attracted her curiosity, on the preceding night, and she resolved to examine it. As she passed through the chambers, that led to this, she found herself somewhat agitated; its connection with the late lady of the castle, and the conversation of Annette, together with the circumstance of the veil, throwing a mystery over the subject, that excited a faint degree of terror. But a terror of this nature, as it occupies and expands the mind, and elevates it to high expectation, is purely sublime, and leads us, by a kind of fascination, to seek even the object, from which we appear to shrink.

Emily passed on with faltering steps, and having paused a moment at the door, before she attempted to open it, she then hastily entered the chamber, and went towards the picture, which appeared to be enclosed in a frame of uncommon size, that hung in a dark part of the room. She paused again, and then, with a timid hand, lifted the
veil; but instantly let it fall--perceiving that what it had concealed was no picture, and, before she could leave the chamber, she dropped senseless on the floor.