The matter is, to some degree, one of symbolic and liturgical common sense, but indeed, taking a very quick look about, other than the mention and recommendation of their use, commentary on the symbolism itself seems rather sparse -- no doubt it was deemed unnecessary.
Quickly peering through ceremonialists like Fortescue and O'Connell, or church arts commentators like Anson and Webb, brought forward no explicit mention of this point on their part, however, a rather obscure little tome, Candles in the Roman Rite by Fr. Edwin Ryan, saw fit to comment on the matter accordingly:
The employment on occasions of sorrow (the Tenebrae, funerals, etc.) of unbleached rather than bleached candles is evidently fitting, since the sombre tones of unbleached wax harmonize with the mournful ceremony, while bleached wax, being far higher in the tone scale, would intrude a note of joy.
This idea is the "common sense" to which I referred. I say it is that because it is consonant with other similarly sombre elements that are mentioned in relation to these liturgical occasions. In other words, there is a consistency with regard to the signs.
The use of unbleached beeswax is mentioned within the context of liturgies for the Dead, inclusive of All Souls Day of course, as well as on the liturgies of Good Friday in the usus antiquior. In other words, the times that, traditionally, black vestments were worn -- also a more sombre sign it goes without saying.
Fr. Ryan further suggests that "[t]he candles at the funeral of a baptized infant are bleached, because that ceremony is clearly intended by the Church to be taken as one of gladness". While I have been unable to find an explicit reference to using bleached candles in this instance, it is noteworthy that, unlike all of the other instances where using unbleached candles is specifically mentioned, here it is absent. Certainly this would make sense, particularly as the vestments worn in these instances are white rather than black in the usus antiquior, as well as the fact that there is a special rite for their funeral where no prayers for the dead are said, and as Fortescue generally comments, "there are no signs of mourning."
We should recall as well that in these same instances a similar sombreness is spoken of even as regards the very candlesticks themselves. Normally the candlesticks we see used in our churches are gold or silver in colour. However, during these times, the mention of these being of some more sombre tone arises as well. Hence we see mention of black or some other darkened colour and the use of iron, dark wood, or bronze.
(Detail of photo by Br. Lawrence Lew)
This is mentioned both in relation to the candlesticks on the altar as well as those which surround the coffin/catafalque -- although, one can certainly see many examples of where this is not done, probably for lack of possessing them.
One will also note that, while it appears to only be specifically mentioned within the context of pre-Pius XII Good Friday liturgy, some also make use of a dark wood altar cross for Masses of the Dead instead of one of gold or silver. (See above and right) This, or some other non-gilt altar cross, would certainly seem to me to be a laudable custom, consonant with this same spirit of sobriety.Finally, it cannot pass mention that relics and other ornaments are not to be used at the altar during these sombre liturgical occasions.
All of these things consistently point to the same theme of a sombre reserve and thus also speak to the symbolic purpose of using unbleached rather than bleached candles.
Now evidently we have been looking at this through the lens of the usus antiquior specifically, but how these would apply to Masses for the Dead in the modern Roman liturgy seems fairly straightforward: unbleached beeswax candles and more sombre candlesticks (and cross) could easily be used at the altar and around the coffin in that context, and of course black vestments used as well.
I would conclude by noting that there are many other sources that could be referenced in looking at this particular question, and so I would invite our readers to feel free to supplement these considerations with their own within the comments.
While we are on the topic of Masses for the Dead, those who use the usus antiquior for these occasions, whether routinely or from time to time, might like to invest in a set of requiem altar cards.
A very nice set that I recently came across whose art is particularly beautiful, and which would look very nice in a dark wood frame for use on the altar, comes by way of Loreto Publications and at a price of $49.95 USD.
