Imam Ali al-Rida

'Alī ibn Mūsā ar-Riḍā (Arabic: علي ابن موسى الرّضا‎), also called Abu al-Hasan, Ali al-Reza (c. 29 December 765 – 23 August 818) or in Iran (Persia) as Imam Reza (Persian: امام رضا‎), was a descendant of the Islamic prophet, Muhammad, and the eighth Shi'ite Imam, after his father Musa al-Kadhim, and before his son Muhammad al-Jawad.

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Seyed Ali Shobayri, Seyed Ali Shobayri is of mixed Iranian and Scottish descent who found the path of the Ahlul Bayt (a) by his own research. He holds a BA in Islamic Studies from Middlesex University through the... Answer updated 2 months ago

Bismillah, 
Asalamu Alaykom, 

Yes Imam Sadiq and the imams after him (peace be upon them) would of been from dark complexion as indicated by narrations. 

We even have authentic ahadith which point towards the 12th imam’s mother being dark
أخبرنا أحمد بن محمد بن سعيد ابن عقدة قال حدثنا محمد بن المفضل بن قيس بن رمانة الأشعري و سعدان بن إسحاق بن سعيد و أحمد بن الحسين بن عبد الملك و محمد بن الحسن القطواني قالوا جميعا حدثنا الحسن بن محبوب الزراد عن هشام بن سالم عن يزيد الكناسي قال سمعت أبا جعفر محمد بن علي الباقر ع يقول إن صاحب هذا الأمر فيه شبه من يوسف ابن أمة سوداء يصلح الله عز و جل له أمره في ليلة واحدة

From Yazeed al-Kunaasee said, I heard Abaa Ja`far Muhammad bin `Alee al-Baaqir (عليهالسلام), he said: “Verily, the master of this affair (i.e. al-Qaa’im) in him is resemblance of Yoosuf (عليهالسلام), he is the son of a black bondswoman (female slave), and Allaah (عَزَّوَجَلَّ) will reform his affair in one night”

Source:
1.     Al-Nu`maani, Kitaab al-Ghaybah, ch. 13, pg. 228, hadeeth # 8
2.     Al-Nu`maani, Kitaab al-Ghaybah, pg. 163, hadeeth # 3

Imam Jawad (as) according to narrations would also be insulted for his darkness and given racist abuse. These realities found in our narrations are very different to what people try to depict the imams as in drawings.

Also Sister Amina Inloes has an interesting article about this topic which may benefit you; google and download the following article entitled: 

'Racial ‘Othering’ in Shi‘i Sacred History: Jawn ibn Huwayy ‘the African Slave’, and the Ethnicities of the Twelve Imams'

May Allah grant you success

Sayyed Mohammad Al-Musawi, Sayyed Mohammad al-Musawi is originally from Iraq and heads up the World Ahlul Bayt Islamic League in London. Other than being involved in various humanitarian projects, he frequently responds to... Answered 2 months ago

There are many narrations that Imam Musa Al-Kadhim (AS) and Imam Ali Al-Redha (AS) were of darker complexion. (Mawsoo'at Al-Mustafa Wal Etrah by Husain Al-Shaakiri, V. 13, P.13)

Imam Al-Jawad (AS)'s mother was from Africa, from the family of Lady Maria, the mother of Ebrahim, son of the Prophet Muhammad (SAWA).

We have many narration that some of his enemies used to call him Al-Aswad (The person with darker complexion).(Tafseer Al-Ayyashi 1: 314.).

We also have narrations that Imam Ali Al-Hadi Al-Naqi (AS) was of darker complexion. (Madeenatul Ma'aajiz by Sayyed Hashim al-Bahrani, V.3, P.504.)

Wassalam.

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Sayyed Muhammad Husaini Ragheb, Sayyed Muhammad Husaini Ragheb has a BA in Law from Guilan University, Iran and has also undertaken Hawzah studies in Qom. He is a Cultural Affairs director of Ethics Group of Al-Mustafa Open... Answered 8 months ago

Sheikh Mufid (r.a) and Ibn Shahr Ashoub (r.a) have only recorded one son for Imam Reza(a.s) and he is Imam Muhammad al-Taqi al-Jawad (a.s) but some other historians have recorded five sons and a daughter for him.
As for Imam Jawad (a.s) the sons are Imam Ali al-Naqi al-Hadi(a.s) and Musa al-mubarqa' who is buried in Qom( Chehel Akhtaran) and he also had a daughter named Hakimah Khatoon who is buried in Samerra,Iraq.

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Sayyed Mohammad Al-Musawi, Sayyed Mohammad al-Musawi is originally from Iraq and heads up the World Ahlul Bayt Islamic League in London. Other than being involved in various humanitarian projects, he frequently responds to... Answered 1 year ago

There ae different opinions among the scholars in this matter.

Al Haafidh Abu Na'eem sated in Hilyat Al-Awliyaa that Ima Al-Ridha had six sons and Jafar is one of them.

Mohammad Ibn Talha al-Shafi'ee had the same opinion. 

Many of our scholars including Shaikh Al-Mufeed and Al-Erbily narrated from Imam Radha (AS) that he wl have only one son (Imam Al-Jawad) and Allah (SWT) will grant this one son lot of sons.

Wassalam