Blanche Sweet is a cultivar of the hyacinth lilac, which is a cross between the common lilac and the Asian lilac S. oblata.  It was selected in 1981 and bears fragrant chalky blue blossoms with a pinkish blush.  It blooms a week or two earlier than the common lilac.

Members of the genus Syringa, commonly known as lilacs, are
shrubs or small trees prized for their showy and fragrant blooms in late
spring. The individual flowers are tubular in form and are borne in
large panicles. While the common name of the plant has come to define a
shade of pale purple, some species and hybrids have pink or white
blossoms. Lilacs begin to set buds for the following year shortly after
they finish blooming; if pruning is desired, it should be done
immediately after flowering to maintain flower production the next year.

Lilacs are native to cooler temperate areas of southeastern Europe
and eastern Asia with winter temperatures below freezing; there are no
lilacs native to North America. The genus includes at least 12 species
and numerous hybrids and cultivars. The Chicago Botanic Garden's
collection contains over 50 varieties of lilac and more than 800 plants.