Śrīla Jīva Gosvāmī, in his notes on this verse, has mentioned how Nārada Muni gave Kaṁsa this information. This incident is described in the Hari-vaṁśa. Nārada Muni went to see Kaṁsa by providence, and Kaṁsa received him very well. Nārada, therefore, informed him that any one of the sons of Devakī might be Viṣṇu. Because Viṣṇu was to kill him, Kaṁsa should not spare any of Devakī's children, Nārada Muni advised. Nārada's intention was that Kaṁsa, by killing the children, would increase his sinful activities so that Kṛṣṇa would soon appear to kill him. Upon receiving the instructions of Nārada Muni, Kaṁsa killed all the children of Devakī one after another.
(Srimad Bhagavatam-----10:1:65-66-----purport).

When the child of Devakī was attracted and transferred into the womb of Rohiṇī by Yogamāyā, Devakī seemed to have a miscarriage. Thus all the inhabitants of the palace loudly lamented, "Alas, Devakī has lost her child!"
"All the inhabitants of the palace" includes Kaṁsa. When everyone lamented, Kaṁsa joined in compassion, thinking that perhaps because of drugs or some other external means, Devakī had undergone this abortion. The real story of what happened after Yogamāyā attracted the child of Devakī into the womb of Rohiṇī in the seventh month of Rohiṇī's pregnancy is described as follows in the Hari-vaṁśa. At midnight, while Rohiṇī was deeply sleeping, she experienced, as if in a dream, that she had undergone a miscarriage. After some time, when she awoke, she saw that this had indeed happened, and she was in great anxiety. But Yogamāyā then informed her, "O auspicious lady, your child is now being replaced. I am attracting a child from the womb of Devakī, and therefore your child will be known as Saṅkarṣaṇa."
(Srimad Bhagavatam-----10:2:15-----translation and purport).

Thereafter, exactly when Vasudeva, being inspired by the Supreme Personality of Godhead, was about to take the newborn child from the delivery room, Yogamāyā, the Lord's spiritual energy, took birth as the daughter of the wife of Mahārāja Nanda.
Śrīla Viśvanātha Cakravartī Ṭhākura discusses that Kṛṣṇa appeared simultaneously as the son of Devakī and as the son of Yaśodā, along with the spiritual energy Yogamāyā. As the son of Devakī, He first appeared as Viṣṇu, and because Vasudeva was not in the position of pure affection for Kṛṣṇa, Vasudeva worshiped his son as Lord Viṣṇu. Yaśodā, however, pleased her son Kṛṣṇa without understanding His Godhood. This is the difference between Kṛṣṇa as the son of Yaśodā and as the son of Devakī. This is explained by Viśvanātha Cakravartī on the authority of Hari-vaṁśa.
(Srimad Bhagavatam-----10:3:47-----translation and purport).

Baladeva was actually the son of Devakī, but He was transferred from Devakī's womb to that of Rohiṇī. This fact was not disclosed. According to a statement in the Hari-vaṁśa:
pratyuvāca tato rāmaḥ
sarvāṁs tān abhitaḥ sthitān
yādaveṣv api sarveṣu
bhavanto mama vallabhāḥ
Gargamuni did disclose to Nanda Mahārāja that Balarāma would be known as Saṅkarṣaṇa because of uniting two families—the yadu-vaṁśa and the vaṁśa of Nanda Mahārāja—one of which was known as kṣatriya and the other as vaiśya. Both families had the same original forefather, the only difference being that Nanda Mahārāja was born of a vaiśya wife whereas Vasudeva was born of a kṣatriya wife. Later, Nanda Mahārāja married a vaiśya wife, and Vasudeva married a kṣatriya wife. So although the families of Nanda Mahārāja and Vasudeva both came from the same father, they were divided as kṣatriya and vaiśya. Now Baladeva united them, and therefore He was known as Saṅkarṣaṇa.
(Srimad Bhagavatam-----10:8:12-----purport).

This is a quotation from Śrīmad-Bhāgavatam (11.5.27). The śyāma color is not exactly blackish. Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura compares it to the color of the atasī flower. It is not that Lord Kṛṣṇa Himself appears in a blackish color in all the Dvāpara-yugas. In other Dvāpara-yugas, previous to Lord Kṛṣṇa's appearance, the Supreme Lord appeared in a greenish body by His own personal expansion. This is mentioned in the Viṣṇu Purāṇa, Hari-vaṁśa and Mahābhārata.
(Sri Caitanya Caritamrta-----2:20:337-----purport).

Śrī Caitanya Mahāprabhu then describes those candidates who are eligible to taste the mellows of devotional service. Their fundamental natures and their varieties are also described. The Lord also informs Sanātana Gosvāmī about all the confidential paraphernalia of devotional service. He gives a description of Goloka Vṛndāvana, where the Lord is engaged in His eternal pastimes described in the Hari-vaṁśa. There is also an opposing description and a favorable description of keśa-avatāra. All these instructions are mentioned herein.
In this way Śrī Caitanya Mahāprabhu blessed Sanātana Gosvāmī, placing His hand on his head. Thus Sanātana received the power to describe these subjects in books like Hari-bhakti-vilāsa.
(Sri Caitanya Caritamrta-----2:23:1-----purport).

In the revealed scripture Hari-vaṁśa, there is a description of Goloka Vṛndāvana, the planet where Lord Śrī Kṛṣṇa eternally resides. This information was given by King Indra when he surrendered to Kṛṣṇa and offered prayers after Kṛṣṇa had raised Govardhana Hill.
(Sri Caitanya Caritamrta-----2:23:116-----translation).

In the Vedic scripture Hari-vaṁśa (Viṣṇu-parva, Chapter Nineteen), there is the following description of Goloka Vṛndāvana:
manuṣya-lokād ūrdhvaṁ tu khagānāṁ gatir ucyate
ākāśasyopari ravir dvāraṁ svargasya bhānumān
svargād ūrdhvaṁ brahma-loko brahmarṣi-gaṇa-sevitaḥ
tatra soma-gatiś caiva jyotiṣāṁ ca mahātmanām
tasyopari gavāṁ lokaḥ sādhyās taṁ pālayanti hi
sa hi sarva-gataḥ kṛṣṇaḥ mahā-kāśa-gato mahān
upary upari tatrāpi gatis tava tapo-mayī
yāṁ na vidmo vayaṁ sarve pṛcchanto ‘pi pitām aham
gatiḥ śama-damāṭyānāṁ svargaḥ su-kṛta-karmaṇām
brāhmye tapasi yuktānāṁ brahma-lokaḥ parā gatiḥ
gavām eva tu goloko durārohā hi sā gatiḥ
sa tu lokas tvayā kṛṣṇa sīdamānaḥ kṛtātmanā
dhṛto dhṛtimatā vīra nighnatopadravān gavām
When the King of heaven, Indra, surrendered to Kṛṣṇa after Kṛṣṇa had raised Govardhana Hill, Lord Indra stated that above the planetary systems wherein human beings reside is the sky, where birds fly. Above the sky is the sun and its orbit. This is the entrance to the heavenly planets. Above the heavenly planets are other planets, up to Brahmaloka, where those advancing in spiritual knowledge reside. The planets up to Brahmaloka are part of the material world (Devī-dhāma). Because the material world is under the control of Devī, Durgā, it is called Devī-dhāma. Above Devī-dhāma is a place where Lord Śiva and his wife Umā reside. Those brightened by spiritual knowledge and liberated from material contamination reside in that Śivaloka.
(Sri Caitanya Caritamrta-----2:23:116------purport).