The mushrikin in Makkah in the time of the Prophet (ﷺ) believed in tawheed ar-rububiya but they did not believe in tawheed al-‘ibadah. Hence they were titled mushrikeen by Allah.

Based on this, can we call the Muslims who do black magic and worship graves and call other people for help other than Allah, inspite of doing Hajj, praying, fasting, etc., as believers?

Yes, those who commit shirk (even if they claim to be Muslims), are considered to be mushrikun just like the mushrikun of Quraysh. Shaykh Muhammad ibn Abdul-Wahhab has used this argument in several of his writings as evidence against those who commit shirk and yet claim to be Muslims.

Allah has forbidden the Muslims from praying, attending funerals of such persons or even to make dua’ for them when their kufr is crystal clear. Allah has said;

“It is not (proper) for the Prophet and those who believe to ask Allaah’s forgiveness for the Mushrikoon, even though they be of kin” [1]

And it was reported that the Prophet (ﷺ) said: “I asked my Lord for permission to pray for forgiveness for my mother, and He did not grant me permission. Then I asked Him for permission to visit her grave, and He granted me permission.” [2]

They cannot be excused by saying that no one came to them who could explain that the things which they did were shirk, because the evidence to that effect in the Qur’an is very clear, and there are knowledgeable people among them whom they could have asked about whether what they were doing was shirk – but they turned away and were content with what they were doing.

And Allaah is the Source of strength. May Allah bless our Prophet Muhammad (ﷺ) and his family and companions, and grant them peace. [3]

References:

[1] Surah at-Tawbah, 9:113

[2] Narrated by Ahmad, 2/441, 5/355, 359; Muslim, 2/671, no. 976; Abu Dawood, 3/557, no. 3234; al-Nasaa’i, 4/90, no. 2034; Ibn Maajah, 1/501, no. 1572; Ibn Abi Shaybah, 3/343; Ibn Hibbaan, 7/440, no. 3169; al-Haakim, 1/375-376, 376; al-Bayhaqi, 4/76

[3] http://www.islamqa.info/en/ref/7867/