In computational creativity, new concepts can be invented through conceptual blending of two independent conceptual spaces. In music, conceptual blending has been primarily used for analysing relations between musical and extra-musical elements in composed music rather than generating new music. This paper presents a probabilistic melodic harmonisation assistant that employs conceptual blending to combine learned, potentially diverse, harmonic idioms and generate new harmonic spaces that can be used to harmonise melodies given by the user. The key feature of this system is the application of creative conceptual blending to the most common chord transitions (pairs of consecutive chords) of two initial harmonic idioms. The proposed methodology integrates newly created blended chords and transitions in a compound probabilistic harmonic space, that preserves combined characteristics from both initial idioms along with those new chords and transitions within a unified setting. This methodology enables various interesting music applications, ranging from problem-solving, e.g. harmonising melodies that include key transpositions, to generative harmonic exploration, e.g. combining major–minor harmonic progressions or more extreme idiosyncratic harmonies.