Daniel S. response is very helpful! I like vitae nigrorum (The lives of black people...) and digna is a good word for the nuance of dignity/importance/matter. I would tweak two things though. Here is how I think it would have been written: Vitae nigrorum dingae sunt.
First, I would change the mood of the "to be" verb sum from Subjunctive to Indicative. The Mood of a verb is simply the attitude of the writer towards the action. Indicative was used whenever a writer considered the action/state of being to be real, i.e. a statement of fact. Thus, the Indicative was used with questions and regular statements. the Subjunctive was used when the writer considered the action/state of being to be possible or unlikely. In English, we show the Subjunctive by applying may/might. I think the reason why Daniel used the Subjunctive here is because the Subjunctive is used in Latin in the First and Third Person (i.e. I / We & He/She/It / They) with commands or exhortation. Generally this is translated "Let us/ Let me..." This is called a Jussive Subjunctive. The problem with this is the Jussive isn't used with forms of the to be verb, especially when describing/defining a state of being such as "matter/dignity." Thus, the Indicative is appropriate and correct in this instance.
Secondly, I would change the Case of the Adjective digna from dignis to dignae. The reason is that with the presence of the "to be" verb sum, this is a clear instance of a Predicate Nominative. A Predicate Nominative is when an Adjective completes the Subject and the Verb. For instance, in this case, Black Lives are ____ is incomplete. The only way to complete the Subject is by applying an Adjective in the Nominative. Thus, digna changes from the Dative singular to the Nominative Plural.
Hope this helps!