Fòram na Gàidhlig

Do the final "n"s in the article blend into the next word as dark/palatized version of themselves, or do they stay as weak "n"s? For example, is the "n" in "fon" in "fon fhòid" dark or weak? And is the "n" in "san" in "san iar..." palatized or weak?

There's a long chapter in Blas na Gàidhlig about this

But yes, in a nutshell any final -n of the definite article (an, nan, dhan, dhen...) assimilates into the next phoneme from a weak /n/ into:

/n/ > /N/ before a/o/u (including fh + vowel)
/n/ > /Nʲ/ before i/e (including fh + vowel)
/n/ > /ŋ/ before g/c

Elsewhere it gets deleted a lot, especially before l n r:
an rathad /ə'Ra.əd/