What species of small black fleas is this?

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What species of small black fleas is this? - briefly
The insect is most likely the cat flea, Ctenocephalides felis, a common small, dark‑colored flea that infests cats and occasionally humans.

What species of small black fleas is this? - in detail
Small, dark-colored fleas are typically members of the order Siphonaptera. Identification relies on size, body shape, combs (ctenidia), and preferred hosts. The most common species fitting this description include:

Ctenocephalides felis (cat flea)

Length: 1.5–3.5 mm, dark brown to black when unmated.
Morphology: Laterally compressed body, two rows of genal and pronotal combs with 4–6 spines each.
Hosts: Cats, dogs, occasional human bites.
Distribution: Worldwide, especially in temperate and tropical regions.

Ctenocephalides canis (dog flea)

Length: 1.5–3 mm, similar coloration to the cat flea.
Morphology: Genal and pronotal combs present, but pronotal comb teeth are generally larger and more widely spaced than in C. felis.
Hosts: Primarily dogs, can bite humans.
Distribution: Global, less common than C. felis.

Pulex irritans (human flea)

Length: 2–4 mm, dark brown to black.
Morphology: No genal or pronotal combs; hind legs are longer than those of cat or dog fleas.
Hosts: Humans, occasionally other mammals.
Distribution: Cosmopolitan, historically linked to human dwellings.

Xenopsylla cheopis (oriental rat flea)

Length: 2–3 mm, dark brown, may appear black after drying.
Morphology: Prominent genal and pronotal combs with 6–8 spines; robust head capsule.
Hosts: Rats, other rodents; can bite humans.
Distribution: Worldwide, especially in urban environments with rodent infestations.

Key diagnostic features to differentiate these species are the presence and arrangement of combs, the shape of the head capsule, and the host association. Microscopic examination of the combs (ctenidia) provides the most reliable identification. If a specimen lacks combs, it is likely Pulex irritans; if combs are present and the pronotal teeth are widely spaced, Ctenocephalides canis is probable; tightly spaced pronotal teeth suggest Ctenocephalides felis; and a robust head with well‑developed combs points to Xenopsylla cheopis.