The extragalactic γ-rays sky observed by the Fermi Large Area Telescope (LAT) is dominated by blazars. In the fourth release of the Fermi LAT Point Source Catalog (4FGL) are sources showing a multifrequency behavior similar to that of blazars but lacking an optical spectroscopic confirmation of their nature, known as blazar candidates of uncertain type (BCUs). We aim at confirming the blazar nature of BCUs and test if new optical spectroscopic observations can reveal spectral features, allowing us to get a redshift estimate for known BL Lac objects. We also aim to search for and discover changing-look blazars (i.e., blazars that show a different classification at different epochs). We carry out an extensive search for optical spectra available in the Large Sky Area Multi-object Fibre Spectroscopic Telescope (LAMOST) Data Release 5 (DR5) archive. We select sources out of the 4FGL catalog, the list of targets from our follow-up spectroscopic campaign of unidentified or unassociated γ-ray sources, and the multifrequency catalog of blazars: the Roma-BZCAT. We select a total of 392 spectra. We also compare some of the LAMOST spectra with those available in the literature. We classify 20 BCUs confirming their blazar-like nature. Then we obtain 15 new redshift estimates for known blazars. We discover 26 transitional (i.e., changing-look) blazars that changed their classification. Finally, we are able to confirm the blazar-like nature of six BL Lac candidates. All remaining sources analyzed agree with previous classifications. BL Lac objects are certainly the most elusive type of blazars in the γ-ray extragalactic sky.

河外γ费米大面积望远镜 (LAT) 观测到的 - 射线天空以耀变体为主。在费米 LAT 点源目录 (4FGL) 的第四版中，源显示出类似于耀变体的多频行为，但缺乏对其性质的光学光谱确认，称为不确定类型的耀变体候选者 (BCU)。我们的目标是确认 BCU 的耀变体性质，并测试新的光学光谱观察是否可以揭示光谱特征，从而使我们能够对已知的 BL Lac 天体进行红移估计。我们还旨在搜索和发现外观变化的 blazars（即在不同时期显示不同分类的 blazars）。我们对大天区多目标光纤光谱望远镜 (LAMOST) 数据版本 5 (DR5) 档案中可用的光谱进行了广泛搜索。γ射线源和耀变体的多频目录：Roma-BZCAT。我们总共选择了 392 个光谱。我们还将一些 LAMOST 光谱与文献中的光谱进行了比较。我们对 20 个 BCU 进行了分类，确认了它们的类似耀变体的性质。然后我们获得了 15 个已知耀变体的新红移估计值。我们发现了 26 个改变其分类的过渡性（即变化外观）耀变体。最后，我们能够确认六个 BL Lac 候选者的类似耀变体的性质。分析的所有剩余来源都与先前的分类一致。BL Lac 天体无疑是γ射线河外天空中最难以捉摸的耀变体类型。