The Blackbird normally nests in the open in shrubbery and does not take an enclosed tit box. If you still want to help it, an open (half-open) box or a sheltered ledge will do.
Type: an open front rather than a round entrance hole.
Internal dimensions: a roomy floor, approx. 18 × 18 cm.
Placement: well hidden in a dense climbing plant or under an eave, 2–3 m up, safe from cats and crows.
In practice, a dense, unclipped hedge or bush is the best "nest site" you can offer a Blackbird.