Praise be to Allah.
In the book al-Taareekh al-Qadeem li Makkah wa Bayt-Allaah il-Kareem (The Ancient History of Makkah and the Noble House of Allaah) by Muhammad Taahir al-Kurdi al-Makki (vol. 3, p. 256), it says that the stone in the Yemeni Corner (al-Rukn al-Yamaani) goes back to the time when ‘Abd-Allaah ibn al-Zubayr (may Allaah be pleased with him) rebuilt the Ka’bah, and that it has remained until our own times, and that everyone who has rebuilt the Ka’bah kept this stone as it was. He says that in 1040 AH, during the time of Sultan Murad IV, who rebuilt the Ka’bah, the edge of the stone of this Corner was broken off, and molten lead was poured in its place, and before that the pieces of the stone had been put together, using adhesive and nails, during the era of the Fatimids.
Perhaps what you saw was that lead or the marks of those nails, with changes in its colour because of perfume or being touched by the hands of people doing Tawaaf.
What is prescribed is to touch this corner, without kissing it or saying Allaahu akbar. If you cannot touch it then you should not point to it, because that was not narrated from the Prophet (peace and blessings of Allaah be upon him).
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:
The Messenger (peace and blessings of Allaah be upon him) used to touch the Yemeni Corner, but he did not say Allaahu akbar. So based on this it is not Sunnah to say Allaahu akbar when touching it.
Al-Sharh al-Mumti’, 7/283.
Shaykh al-Albaani (may Allaah have mercy on him) said:
(The pilgrim) touch the Yemeni Corner with his hand in each circuit (of Tawaaf), but he should not kiss it. If it is not possible to touch it, then he should not point to it with his hand.
Manaasik al-Hajj wa’l-‘Umrah, p. 22
Touching it is indicated by the hadeeth narrated by al-Haakim from Ibn ‘Umar, according to which, when the Messenger of Allaah (peace and blessings of Allaah be upon him) circumambulated the Ka’bah (Tawaaf), he would touch the Black Stone and the Yemeni Corner in each circuit.
Saheeh al-Jaami’, no. 4751.
Concerning the virtue of touching the Yemeni Corner, the Prophet (peace and blessings of Allaah be upon him) said: “Touching the Black Stone and the Yemeni Corner indeed erases sins.” (Narrated by Ahmad from Ibn ‘Umar; classed as saheeh by al-Albaani in Saheeh al-Jaami’, no. 2194).
And Allaah knows best.