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Violet-black cells in gelatinous sheaths at the surface of a layer of gelatinous cyanobacteria, growing on a lost sock in the mud of a flooded abandoned limestone quarry. I'm not certain that these photos are all of the same organism - some are dark violet and some are more strongly just black. The violet capsule absorbs so much light that it is difficult to induce autofluorescence in the chlorophyll of the cell inside. The first picture shows the violet layer beside some Trentepohlia. The various pictures demonstrate the colours in contrast to some of the other species present (Hassallia, Trentepohlia). Cells themselves are 3-6 µm and the capsules are 6-12 µm. The surface violet-black layer covers a deep layer of gelatinous cells inside lamellated mucilaginous sheaths, of different generations, that I now think are another species entirely.
Part of a gelatinous community: https://inaturalist.ca/observations/208411876.

Cryptomonas (Chilomonas) paramecium Ehrenberg 1832. A bloom from the acidic freshwater kettle pond Chatfield's Hole fed with boiled wheat seeds. Imaged in Nomarski DIC on Olympus BH2S using SPlan 100 1.25 oil objective plus variable phone cropping on Samsung Galaxy S9+. As Jara Kubin points out, the correct name after molecular analyses is Cryptomonas paramecium, the species is a regular member of the genus, but without chlorophyll.

Hoef-Emden, K. & Melkonian, M. (2003). Revision of the genus Cryptomonas (Cryptophyceae): a combination of molecular phylogeny and morphology provides insights into a long-hidden dimorphism. Protist 154: 371-409

Genus: Similar to Cryptomonas in general body form and structure, but colorless; without pyrenoid; "cytopharynx" (gullet) deep, lower half surrounded by granules (ejectisome/trichocyst); one contractile vacuole anterior (Kudo, 1966). Chilomonas does not have chromatophores (pigment-containing structures) and lives by ingesting organic matter.
Species: 20-40 μm long. Posteriorly narrowed; slightly bent 'dorsally'; 30-40 μm long, 10-15 μm wide; widely distributed in stagnant water (Kudo, 1966).

Free-swimming, biflagellate monads, generally obovoid often with a recurved posterior sometimes produced to acutely pointed; not known to produce palmelloid colonies. A longitudinal furrow extends posteriorly from the vestibulum and transforms into a sack-like gullet lined with many rows of ejectosomes. With a single nucleomorph but lacking a chloroplast and pyrenoid; with a prolific accumulation of peripheral starch granules. Periplast with a laminate inner component and a superficial layer of fine fibrillar material. Sexual reproduction is unknown; reproduction being through simple cell division. Cyst formation is unknown.

Part of a mixed community of cyanobacteria and lichens that have colonized a sock, embedded in the mud of a flooded abandoned quarry. The filaments of Scytonema are embedded in thick yellow sheaths, 12 to 20 microns wide. The epifluorescence photos reveal the form of the filaments within the sheaths.