Skin blisters when it gets very hot, or is badly burned.  So, in this sentence, blisteringly hot means it could be so hot that it feels like a person’s skin could be badly burned.　

[氣象人員預測，今年夏天比往常還要熱，會有酷熱無比的日子。]

句中 a warmer than average summer 是「比普通夏天更熱的夏天」；blister若作動詞用，可指皮膚因受傷或過熱而起水泡（或氣泡），如今拿blisteringly hot來形容天氣，當然是「熱到不行」囉！