While most interpretations of this poem that one finds online describe the work as clearly relating to a mother and daughter, I find myself wondering if it relates to a different theme.
The poem was released in a 1972 collection, putting it in the wake of Angelou's own soujourn in Egypt and Ghana from 1961 to 1965. She first went to Egypt with her then partner Vusumzi Make, but with her son, Guy, moved to Ghana after breaking up with make.
Ghana at that time ran a programme encouraging African-Americans to move there. (Ghana had recently become the first colony in the Sub-Saharan region of Africa to gain its independence from European colonial powers under the leadership of its first president, Kwame Nkrumah -Wikipedia: African-Americans in Ghana)
In light of this, there is a possible reading of the poem as an autobiographical work where 'she' represents Angelou and the Mothering Blackness represents Africa.
In this reading the alien dreams may be seen as the nightmare of the slave trade itself, taking ancestors to an alien land. ‘Rime’ would have the meaning of the salt of dried tears looking like a layer of frost, dried tears being appropriate to recovering from a nightmare and the idea of frost being a contrast to the welcoming heat of sub-Saharan Africa.
The reference to both the slave trade and freedom is emphasised in the third verse, Hagar was a slave, taken out of her own country and then cast aside with her son. Sheba was a Queen who returned, by some stories, to her own land to bring up her son alone. This may be a metaphor for Angelous' break up with Make and arrival in Ghana with her son.