Sky appears like congealed blood
Imam Baihaqui (May Allah shower His mercy on him) writes in his book Dalaail Un Nubuwwa:
Translation: Hadhrat ‘Ali bin Mashar narrates on the authority of his grandmother who said: In the era of Imam Hussain (May Allah be well pleased with him), I was a young girl. In that time, the sky become like congealed blood for many days. (Dalaail Un Nubuwwah, Hadith No. 2812)
Allah’s displeasure and anger was expressed in many ways. That the sky became blood-red is also mentioned. Similarly, it is established through many Traditions that the sky became red.
When human beings get angry their faces turn red with anger. Allah Most High is beyond having a physical body. He expressed His displeasure and anger by reddening the sky. As mentioned by Hadhrat Ibn Hajar Haitami (May Allah shower His mercy on him) in As Sawaaiq Al Muhriqa:
Translation: Ibn Jauzi said: The wisdom behind reddening of the sky is that when we get angry, our anger is expressed by flushing of our face. The Lord Almighty is above and beyond having a physical body. That’s why on the martyrdom of Imam Hussain (May Allah be well pleased with him), Allah Most High expressed His displeasure by making red so that everybody knows how grave this crime is. (As Sawaaiq Ul Muhriqa, Pg. No. 116)