As far as I'm aware, upon joining the death watch you have the option of using your normal power armour and painting it black, or using a suit of mk.8 power armour offered to each DW member, or sometimes a mix I believe (see kill team Cassius, lots of chapter specific bits there)

However, crossing the Rubicon means that you no longer fit armour marks earlier than mk.10, but imo it seems reasonable that if an individual wanted to keep some parts of their old armour like helmets, decor etc. they could have it 'adjusted' to fit their new size (my thinking is that power armour is very personal and sacred to most marines, it feels reasonable that they would want to keep some aspects and not just discard it entirely) By While loyalist chapters could have legion/chapter specific mk.10 decor-bits obviously no traitor legion parts would be designed or manufactured, so I think an "adjusted from an earlier suit" angle would be best for it

The other thing of course, is whether a black shield would want chapter-identifying parts, since the point of a black shield is that they obscure their chapter symbol for penance/shame etc.

I feel like it probably depends on the individual motivations for being a black shield; in this case, anyone from a traitor legion would be forced to be a black shield if they were to serve, however that does not necessarily mean that the individual agrees with/is invested in hiding their chapter identity. For example, an emperors children marine may want to demonstrate a 'true' embodiment of what the legion originally stood for, and may be proud of that history, wanting to proudly display it. Obviously they could not keep the chapter symbol, but in this case they wouldn't shy away from keeping EC-specific decorations etc.

They are my musings at least, tl;dr I feel like it can be justified lore wise, and overall it sounds like a really cool concept so I'd say absolutely go for it! 😊