It looks like a scratch on the toenail

Longitudinal melanonychia also designated melanonychia striata is a melanin-derived brown or black pigmentation along the nail.^1,3 The pigmented streak is caused by a melanin deposition that runs from the proximal nail fold to the distal part of the nail plate.^2,7,9
It looks like a dark band within the nail plate from the proximal boarder up to the distal margin.
Longitudinal melanonychia can affect a single nail, as in the clinical case presented or multiple nails which is less frequent.² This alteration is often located on a fingernail and the thumb is the most common site of occurrence.^3,6,7,8,10 On feet generally, as it is more prone to trauma, the hallux is more frequently affected than the other toes.⁷ But it can occur regardless of the existence of trauma, as seen in the clinical case presented.
In general, longitudinal melanonychia result from an increased activity of melanocytes in the nail matrix, with subsequent increased synthesis of melanin in the nail matrix and deposition in the nail plate.^2,7 Causes of longitudinal melanonychia are related to melanocytic activation and include ethnic melanonychia in dark-skinned individuals, pregnancy, chronic local trauma, infections, graft-vs-host disease, hemochromatosis, porphyria, alkaptonuria, Peutz-Jeghers and Laugier-Hunziker syndromes, dermatological disorders, endocrine disorders and certain medications.^6,7
Longitudinal melanonychia can also result from melanocytic hyperplasia, which seems to be more frequent in children due to a melanocytic nevus or lentigo affecting the nail matrix.^4,7,10 In the pediatric population a study of 40 cases showed that benign melanocytic hyperplasia (lentigo or nevus) was the cause of 77.5% of cases of longitudinal melanonychia and of 85% of cases in the subset of white patients.⁴
The differential diagnosis includes exogenous discoloration, splinter hemorrhages, longitudinal erythronychia, subungual hematoma, fungal melanonychia, onychomatricoma, transverse orange-brown chromonychia, atypical melanocytic hyperplasia and melanoma of the nail matrix.^3,6,7
The prevalence of longitudinal melanonychia varies significantly, ranging from 1.4% in fair-skinned populations, where it is rare, to 77% in dark-skinned populations.¹ No differences in sex incidence had been reported.¹⁰ This case is atypical because this condition is rare in caucasian individuals and even more in children.^6,7 So in children diagnosis and management criteria of longitudinal melanonychia are even more poorly defined.¹⁰
The diagnosis is mainly clinical and usually done by a dermatologist.⁷ In the absence of concerning clinical features such as a pigment band wider than 3 mm, rapid growth, increasing darkening of the lesion, pigment variegation, periungual pigmentation (Hutchinson sign), nail plate dystrophy or a family history of melanoma or dysplastic nevus-biopsy and histopathologic examination should be avoided to prevent the risk of permanent nail dystrophy.^1,3,5,7 For this reason, pediatricians should be familiar with this diagnosis and aware of the importance of referring for evaluation by a dermatologist.
In most cases, longitudinal melanonychia is a benign condition, particularly in children and subungual melanoma is exceptionally rare in this age group.^1,2,4 In a study involving 40 children with melanonychia, no malignant lesions were detected upon histologic examination.⁴ Because of its typically indolent and non-aggressive clinical course, nearly all cases can be managed conservatively. Most researchers recommend a watchful waiting approach, as was the case in our situation, which continues to be monitored clinically.^1,3,4,7,8
Spontaneous regression of longitudinal melanonychia, despite having been described, is rare and unique to children.^2,8,10
Extended follow-up is recommended once melanoma of the nail apparatus, in adults with a previous history of longitudinal melanonychia beginning in childhood, has been reported.^1,3,4,7,10

References :

Burleigh A, Lam JM. Pediatric longitudinal melanonychia. CMAJ. 2017; 189:e1093. Doi: 10.1503/cmaj.170256 Chu DH, Rubin AI. Diagnosis and management of nail disorders in children. Pediatr Clin North Am. 2014;61(2): 293-308. Doi: 10.1016/j.pcl.2013.11.005 Cooper C, Arva NC, Lee C, Yélamos O, Obregon R, Sholl LM, Wagner A, et al. A clinical, histopathologic, and outcome study of melanonychia striata in childhood. J Am Acad Dermatol. 2015;72(5):773-9. doi: 10.1016/j.jaad.2015.01.010 Goettmann-Bonvallot S, André J, Belaich S. Longitudinal melanonychia in children: A clinical and histopathologic study of 40 cases. J Am Acad Dermatol. 1999; 41(1):17-22. Doi: 10.1016/s0190-9622(99)70399-3. Gomes S, Lencastre A, Lopes MJP. Alterações ungueais em Pediatria. Nascer e Crescer 2012; 21(1):19-24. Guerrero-Fernández J, García-Ascaso MT, Guerrero Vázquez J. Pigmentación en banda en uña del pie. An Pediatr (Barc) 2004; 61(5):455-6. Doi: 10.1016/S1695-4033(04)78431-3 Leung AKC, Lam JM, Leong KF, Sergi CM. Melanonychia striata: clarifying behind the Black Curtain. A review on clinical evaluation and management of the 21st century. Int J Dermatol. 2019; 58(11):1239-1245. Doi: 10.1111/ijd.14464 Matsui Y, Sasaki J,Sumiko Takatsuka S, Tatsuya Takenouchi T. Natural course of pediatric longitudinal melanonychia: A retrospective cohort study in Japan. J Am Acad Dermatol 2021; S0190-9622(21):00644-7. Doi:10.1016/j.jaad.2021.03.082 Richert B, André J. Nail disorders in children: Diagnosis and management. Am J Clin Dermatol. 2011;12(2):101-12. Doi: 10.2165/11537110-000000000-00000. Starace M, Piraccini BM, Brandi N, Alessandrini A. When the nail appearance plays tricks: a case of longitudinal melanonychia. EMJ 2018; 3(1):106-110.