Her menses stopped years ago, then she saw dark blood; is this regarded as menses?
Question

My menses stopped three or four years ago, but suddenly, two days ago, I found a dark discharge (like the discharge at the beginning of menses) in the vagina, but it was only inside.
Today I checked, and the cloth came out with a very small amount of red blood, then the black discharge, but it did not come out; I found it when I checked with a piece of cloth.
My question is: should I stop praying, or should I do ghusl and offer the prayers that I missed for the last two days, when I first found it (because I did not pray when I found it)?
Praise be to Allah.Firstly:
The basic principle is that the blood that comes out of a woman is menstrual blood, even if her menses had stopped for several years, or she has reached the age of fifty or older, because there is no defined age for menopause, according to the more correct scholarly view. Allah has connected the rulings on menses to the presence of adha (“harm”), which refers to the blood, so when there is blood, it is assumed to be menses, unless the bleeding lasts for longer than fifteen days, in which case it becomes istihadah (non-menstrual bleeding).
Allah, may He be exalted, says (interpretation of the meaning):
{And they ask you about menstruation. Say, "It is harm, so keep away from wives during menstruation. And do not approach them until they are pure. And when they have purified themselves, then come to them from where Allah has ordained for you. Indeed, Allah loves those who are constantly repentant and loves those who purify themselves} [al-Baqarah 2:222].
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: There is no specified age after which women stop menstruating. Rather, even if we assume that a woman sees the usual type of blood coming from the womb after the age of sixty or seventy, it is to be regarded as menses. The menopause mentioned in the verse (interpretation of the meaning) {And those who no longer expect menstruation among your women} [al-Talaq 65:4] is not referring to a particular age. If it was referring to a particular age, then Allah and His Messenger would have explained that. Rather the verse is speaking of a woman who thinks that she will no longer menstruate. Once her menses stops and she no longer expects it to return, then she “no longer expects menstruation,” even if she is forty years old. Then if she spent some time without menses, and the bleeding comes back, then it becomes clear that she had not reached menopause."(Majmu‘ al-Fatawa 19/240).
Secondly:
Menstrual blood may be black, red or brown.
It was narrated from Fatimah bint Abi Hubaysh that she suffered from istihadah (irregular menstrual bleeding), so the Prophet (blessings and peace of Allah be upon him) said to her: “When it is menstrual blood, it is dark and recognisable, so when that happens, stop praying…” Narrated by Abu Dawud (286); classed as hasan by al-Albani in Sahih Sunan Abi Dawud.
Al-Khatib al-Sharbini said: “Its colour” that is, of the strong blood, “is black”, then red, which is weak in comparison to the black blood, and is strong in comparison to the brown. The brown is stronger than the yellow, which is stronger than the brownish colour. That which has an unpleasant smell is stronger than that which has no smell, and that which is viscous is stronger than that which is thin and black."(Al-Iqna‘  1/95).
Thirdly:
If the blood is inside the vagina, then it is menses, and there is no stipulation that it should come out. The presence of blood inside indicates that something has come out from the womb, which is menses.
Ibn Abi Shaybah narrated in al-Musannaf (1/90) with his isnad that ‘Amrah bint ‘Abd al-Rahman al-Madaniyyah (d. 98 AH) – one of the female jurists of the generation of the Tabi‘in – used to say to the women: If one of you inserts a piece of cloth and it comes out changed, then do not pray until you do not see anything.
It says in Matalib Uli al-Nuha (1/166): “Similar to that” – that is, the movement of semen – “is the movement of menstrual blood.” This was stated by Shaykh Taqiy al-Din. This ruling applies when sensing its movement inside the body as when it comes out. So if the woman feels the movement of menstrual blood just before sunset, and she is fasting, then her fast is broken, even if the blood does not come out until after sunset. End quote.
Ibn Hajar al-Haytami (may Allah have mercy on him) said: Blood inside the vagina – which is blood that does not come out when squatting on one’s heels – is menses, if that occurs at the time when menses is due…
The ruling concerning menses and the rulings that result from its presence are connected to awareness of the presence of blood in the vagina at the time when it is possible for menses to occur, so that if it is present, that time is deemed to be menses, otherwise it is not…
Menses comes and goes, so when the blood is present in the vagina, then the rulings on menses come into effect, even if it does not come out."(Al-Fatawa al-Fiqhiyyah al-Kubra  1/76).
Based on that, you did well by not praying when you were menstruating. And your menses continues until you see that you have become pure with the cessation of bleeding, or dryness, or the white discharge, unless the bleeding lasts for more than fifteen days, after which you should do ghusl, pray and act in accordance with the rulings on istihadah.
And Allah knows best.