Blue naevus was first described by Tiesche in 1906 . These lesions may cause diagnostic difficulty  in case of large size, involvement of subcutaneous tissue, asymmetrical pigmentation and  presence of lymphnode metastasis.

Unlike benign melanocytic naevi, in blue naevi: 1) there  is no evidence of diminution in cellularity and cell size towards the base of the lesion  2) no associated  loss of pigmentation 3) no loss of proliferative activity.
The macroscopic bluish appearance is due to the presence of deep intradermal melanin pigment viewed through intact skin ( Tyndall effect) 

Site:  Although it usually occurs in skin, it has been reported in other locations, such as oral mucosa, sclera, uterine cervix, vagina, prostate, spermatic cord, pulmonary hilus, orbit, conjunctiva, maxillary sinus, breast, and lymph nodes.

Images: (DermAtlas):    

Histologically, these are ill-defined proliferation of elongated , sometimes finely branched dendritic, finely pigmented melanocytes within the dermis.Variable numbers of melanin laden macrophages (melanophages) containing large aggregates of melanin are present.  A Grenz zone  usually separates the lesion from the epidermis.
There may be increased cellularity and pigmentation close to the adnexae or along nerves and blood vessels.
Occasionally, the lesion is hypomelanotic (D/D dermatofibroma).
Usuall differential diagnosis include nevi of Ito, Ota and the Mongolian spot.   Image link

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Arthur Allan in 1949 for the  first time recognised that this larger,  cellular and often mitotically active melanocytic lesions were actually benign.
These acquired lesions are usually  located in the sacrococcygeal/gluteal  region and less common sites include face scalp and distal extremities.
These lesions clinically presents as nodule , tumour or plaques, usually 1 - 2 cm in diameter
(sometimes much larger).
Microscopically, this is a symmetrical lesion composed of  two distinct cell types, dendritic  melanocytes as in the common type, together with islands of plump , oval melanocytes with abundant cytoplasm. The plump cells have  a round or oblong nucleus and central nucleolus. Often intranuclear cytoplasmic pseudoinclusions are present .
Often mitotic figures are noted  (less than  2/sq mm). No atypical mitotic forms are present.
The deeper portions are often well circumscribed and can extend into the subcutis.
The growth pattern may be diffuse or nested . The terms used to describe the  nested pattern include  zellballen, fascicular  and alveolar.
The differential diagnosis include  malignant melanoma, fibrous histiocytoma, pigmented dermatofibrosarcoma protuberans and schwannoma.
IMAGE LINK:      

CD34-positive cellular blue nevi.J Cutan Pathol. 2001 Mar;28(3):145-50.       

Epithelioid blue nevus of the oral mucosa: a rare histologic variant.Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2003 Oct;96(4):429-36.

ATYPICAL CELLULAR  BLUE  NAEVUS:
Proposed by Mihm et al, the lesion demonstrates clinico-pathological features intermediate between cellular blue naevus and malignant blue naevus. There is evidence of architectural and cytological atypia together with necrosis. No atypical mitoses were present and there was no evidence of  metastasis.

"Atypical" blue nevus, "malignant" blue nevus, and "metastasizing" blue nevus: a critique in historical perspective of three concepts flawed fatally.Am J Dermatopathol. 2004 Oct;26(5):407-30.

The lesion includes clear cut areas of malignant and  benign blue naevus, or the histological appearances may resemble cellular blue naevus together with pleomorphism prominent atypical mitoses and areas of coagulative necrosis.  Follow up of the cases reveal significant morbidity and death by metastatic disease.

Melanoma associated with blue nevus and melanoma mimicking cellular blue nevus: a clinicopathologic study of 10 cases on the spectrum of so-called 'malignant blue nevus'.Am J Surg Pathol. 2001 Mar;25(3):316-23.

Syndrome is characterised by multiple pigmented lentigines of skin and mucous membrane  (eg. epitheliod type blue naevi  or usual type of blue naevi) , cardiac myxoma, endocrine overactivity syndromes and neurinomas including psammomatous melanotic schwannoma.
Epithelioid blue nevus and psammomatous melanotic schwannoma: the unusual pigmented skin tumors of the Carney complex.Semin Diagn Pathol. 1998 Aug;15(3):216-24.

Blue Naevus with Lymphnode metastasis:

Blue naevus with lymphnode metastasis is characterised by cellular blue naevus in the skin associated with the finding of melanocytes within a regional lymphnode .
In contrast to melanoma cells, blue naevus cells within lymphnodes are confined to capsule and do no show atypical features or mitosis. It has been suggested that the presence of cells of a blue naevus within a lymphnode is due to error during migration of melanocytes from the neural crest to the skin.

Plaque-type blue nevus of the oral cavity.Dermatology. 2005;211(3):224-33.

Malignant blue nevus with lymph node metastases in five-year-old girl.Croat Med J. 2005;46(3):463-6.

Congenital pauci-melanotic cellular blue nevus.J Cutan Pathol. 2004 ;31(4): 312-7.

Childhood malignant blue nevus of the ear associated with two intracranial melanocytic tumors-metastases or neurocutaneous melanosis?Hum Pathol. 2004 Oct;35(10):1292-6.

Eruptive multiple blue nevi of the penis: a clinical dermoscopic pathologic case study.J Cutan Pathol. 2004 Feb;31(2):185-8.

Amelanotic cellular blue nevus: a hypopigmented variant of the cellular blue nevus: clinicopathologic analysis of 20 cases.Am J Surg Pathol. 2002 ;26(11):1493-500.

"Compound blue nevus": a reappraisal of "superficial blue nevus with prominent intraepidermal dendritic melanocytes" with emphasis on dermoscopic and histopathologic features.J Am Acad Dermatol. 2002 Jan;46(1):85-9.

Epithelioid blue nevus: neoplasm Sui generis or variation on a theme?Am J Dermatopathol. 2000 Dec;22(6):473-88.

Large plaque-type blue nevus with subcutaneous cellular nodules.Am J Surg Pathol. 2000;24(1):92-9.

Blue nevus: classical types and new related entities. A differential diagnostic review.Pathol Res Pract. 1994;190(6):627-35.

Prognostic parameters of melanoma

Lentigo maligna melanoma

Superficial spreading melanoma

Nodular melanoma

Acral lentiginous melanoma

Desmoplastic /Spindle cell /
Neurotropic melanoma

Naevoid melanoma

Balloon cell melanoma

Extra-adrenal Paraganglioma