Isabella of France was almost certainly blonde
Kathryn Warner has, over the years, made several quite categorical remarks about our ability to reconstruct Isabella of France's appearance. In 2013, she stated that "(n)o source records Isabella’s appearance, other than that she was very beautiful... We have no idea whether her hair was blonde or black or chestnut brown or auburn, whether she was slender or plump, short or tall, pale-skinned or darker, whether she had blue or green or hazel or brown eyes." Having looked at the available evidence, however, I've concluded that this degree of agnosticism is unwarranted. We actually are able to reconstruct, with a fair degree of certainty, aspects of her appearance, and one of those aspects is her hair colour. There are two main pieces of evidence I wish to present in this regard:
1. Isabella is explicitly referred to as being blonde in at least one English source dating from her lifetime.
And it is a very famous source indeed, one you would have almost certainly encountered before if you have explored the historiography of Edward II's life and reign before: the Lament. The relevant description occurs in the eighth stanza:
Parti m'ount un ju saunz joie;
de tel tristour my qoer se pleint
De cele est qe trover quidoye
fyn loialté, vers moi se feint.
Dieux! come bien amay la bloye;
mais ore est l'estencele esteint
fyn amour; pur ceo ma joie
s'en est alee, come est de meint. *
The fifth line reads, quite clearly, as "Oh God, how much did I love the blonde one!" Those who are familiar only with Paul Studer's translation of the poem, in which the relevant word is rendered simply as "fair", might assume it simply means "fair" in the sense of "beautiful" or "comely." Not so. La bloye is, unambigously, a reference to hair colour. When used as a noun-phrase, the Anglo-Norman dictionary explicitly translates bloi as "blond-haired person." It is true that blond hair, or crine bloie, was an element of courtly descriptio, one of the set of conventional features with which the domnas of courtly love poetry or the heroines of chivalric romances were described with as a matter of course. But there is an obvious difference between giving a fictional female character a specific hair colour in line with contemporary beauty standards and describing a famous living individual, who the reader is supposed to be able to identify, with a hair colour she did not have. If Isabella had had an unflattering hair colour, the author could simply have not made any mention to it at all. The Chronique métrique of Geoffrey of Paris describes Isabella as being “la plus belle des belles", so, to me, the most parsimonious explanation of the poem's phrase is that Isabella was indeed blonde, and this was one of the ways in which her appearance aligned with the beauty standards of the day.
There's an additional point to be made about the language of the poem itself. Anglo-Norman courtly love poetry would have had a readership that skewed heavily towards the societal elite. The people of England outside of the orbit of court still overwhelmingly spoke English. A substantial portion of the people who could even read this poem would have also seen Isabella in person on public occaisons. It remains, of course, possible this description was a mere idealisation with no relation to her actual appearance, but I really do not find that particularly probable.
2. Her father was explicitly described as blond too, and Geoffrey of Paris draws a direct connection between Isabella and her father's good looks
Philip IV is explicitly described, in an anonymous royal chronicle written c. 1342, when he was definitely still in living memory, as having been flavus, rubicundus et candidus et decorus; which Elizabeth A. R. Brown translates as “golden-blond, ruddy, fair, and seemly." While this may not seem all that relevant to Isabella, given that genetic inheritance is not deterministic, we do have a contemporary source that gives a fairly strong indication that Isabella did indeed resemble her father, and it is, once again, the Chronique métrique. Right after describing the dramatic incident in which Edward II saved Isabella's life after a fire broke out in their silken pavillion in Pontoise, Geoffrey of Paris, apropos of nothing, writes:
La noble et sage dame Ysabiau,
Son père des biax le plus biau;
Si est bien droite égalité,
Que bel engendroit grant beauté.
Which I would translate as:
"The noble and wise lady Isabeau,
her father, the fairest of the fair;
It is only right and proper,
that so fair a man should beget such beauty."
Notice that Geoffrey is using the grammatically masculine version of the same phrase he used previously for Isabella: the fairest of the fair. This is Geoffrey stating, in no uncertain terms, that Isabella got her good looks from her father. If her father was blond, as we have fairly good reason to believe, we have a good reason for taking the description of her in the Lament at face value. As much as it bums me out, as a Raven-haired Isabella headcanoner, I see no reason to doubt the preponderance of evidence here.
*As a side note, Studer translates the relevant line as "Fair Isabel I dearly loved" based on the version of the poem found in Longleat Manuscript 26, in which the line reads "beux tant amay la bloye." I have instead used the version of the poem found in the Royal manuscripts, in which the line reads as above. Studer and Aspin take "beux" as being a hypocorism of "Isabeaux" and translate it as "Isabel" on that basis. I, however, concur with Smallwood's critcism of this choice: "Studer's emendation from MS Beux or Deux relied chiefly on romantic appeal, and R's line must be preferred. It has never been shown that Beux was a known pet-form of Isabel."