it's pretty much become nothing now, since afaik I'm the only person with Gaelic who still checks the forum occasionally.
for starters, ch never reduces, only dh/gh. it's about position: broad dh/gh "reduces" (this changes vowel quality and sometimes quantity, but the specifics of that vary by dialect) between a vowel and a consonant (e.g., adhbhar reason, bliadhna year, and I can't actually think of any examples of broad gh in this position, although there are undoubtedly one or two around).
between vowels, broad dh/gh sometimes indicates a hiatus (adha liver, bodhar deaf, e.g.), but also some dialects pronounce words like adha as monosyllables ([a.a] vs. [a:], exact vowel quality varying).
also, it's ["bl(_j)iana], not *["bl_jana], and also the vowels aren't actually [a], but I've never been good at vowel transcriptions and the actual realization varies by dialect anyway. generally speaking, orthographic i is always prononced if it's the first vowel in a series.
tha dannsa nad ghluasadan,
’s bàrdachd neònach air cùl do bhruidhinn,
anns na faclan nach abair thu idir.