The count - non-count distinction has grey areas. Determiner use doesn't seem an infallible test.
In 'there was an eerie silence', which few would render 'there was eerie silence' [see these Google Ngrams], one wonders how one should really define countable as 'there were two eerie silences' sounds very strained.
One can have flu or the flu, but 'a flu' is not unknown.
The use of 'a' before a noun is not seen as a sufficient marker for a count usage by say CGEL. So
- He took a pride in his appearance
is not seen as a count usage (*'He/They took 2/17/several/half a dozen prides in ...').
- The state gave me a good education
(but not The state gave me two good educations).
- I have an understanding of the principles involved
but not I have two understandings ....
but not They felt two strange warmths.
I'd say 'cast a blinding light on' and 'cast blinding sunlight on' are both notionally mass usages. Yes, the first takes the indefinite article, but not numerals. Perhaps the complication comes about as instances are countable (eg spells of sunlight), but light (in the sense used here) itself is not, and the two notions are somewhat conflated.
I've not been able to find any really good articles dealing with this use of the indefinite article with what seem to be better classed as mass rather than count noun usages (though notionality cannot be used as a test here [eg furniture; referent discrete, but almost entirely used in non-count usages]): these nouns fail the 'no a/an' test for massness but pass the 'no numerals' test. The problem is highlighted in the Indefinite vs. zero article before (modified) noncount nouns? thread at English Forums. CalifJim hazards what he concedes are guesses at analyses, but 'a gentle light suffused the scene' isn't in a semantic class he addresses.
In an article at Useful English is found:
In formal writing and literary works the article a/an may be used with
some uncountable abstract nouns to show an unusual or temporary aspect
of something. The indefinite article here has similar meaning to:
such, certain, special, peculiar.
Compare these examples.
Formal / literary style:
The director spoke at the meeting today with an enormous enthusiasm.
A paralyzing horror overwhelmed him.
She smiled at us with an unusual friendliness.
Standard / everyday style:
The director spoke at the meeting today with great enthusiasm.
He was paralyzed with horror.
She smiled at us with unusual friendliness.
It makes sense to me. So the 'rule' indefinite articles cannot be used with mass nouns isn't strictly true. However, just which uncountable abstract nouns one 'may [use an indefinite article] with, to show an unusual or temporary aspect of something' isn't spelled out. Apparently, 'sunlight' isn't one of them.