| Blackrock Administration Ltd | Administrative & support | 2025 |
| Blackrock Advisors Limited | Administrative & support | 2012 |
| Blackrock Construction Ltd | Construction | 2019 |
| Blackrock Developments Limited | Construction | 2025 |
| Blackroom Studios Limited | Other services | 2018 |
| Blacksky Limited | Transport & logistics | 2015 |
| Blackson Limited | Administrative & support | 2018 |
| Blackspot Sport Ltd | Wholesale & retail | 2021 |
| Blackstock Services Limited | Financial services | 2016 |
| Blackstone Euroasia Ltd | Wholesale & retail | 2018 |
| Blackstone Labs Limited | Wholesale & retail | 2020 |
| Blackstone One Ltd | Financial services | 2011 |
| Blackstone Outsourcing Limited | Professional & scientific | 2021 |
| Blackstone Plinth Estates Ltd | Real estate | 2019 |
| Blackthorn Advisors Ltd | Financial services | 2025 |
| Blackthorn Performance Cars Ltd | Wholesale & retail | 2025 |
| Blackthorn Ventures Holding Ltd | Financial services | 2025 |
| Blackwater Gallery Limited | Wholesale & retail | 2024 |
| Blackwater Media Ltd | Information & communication | 2025 |
| Blackwood Embedded Solutions Limited | Information & communication | 2008 |
| Blackwood International Freight Ltd | Wholesale & retail | 2025 |
| Blade Yachting Ltd | Extraterritorial | 2025 |
| Blades & Fades Ltd | Other services | 2018 |
| Bladez&Beauty Ltd | Other services | 2018 |
| Bladon Inns Ltd | Hospitality | 2025 |
| Blaenau Isaf Farm LLP | | 2025 |
| Blaencothi Energy Park Limited | Energy | 2021 |
| Blaenllechau Properties Limited | Real estate | 2016 |
| Blago Limited | Transport & logistics | 2018 |
| Blaidd Consultancy Limited | Information & communication | 2018 |
| Blain Media Communications Ltd | Information & communication | 2015 |
| Blaina Ltd | Transport & logistics | 2021 |
| Blair's Soft Play Ltd | Hospitality | 2021 |
| Blaise Rose Limited | Construction | 2018 |
| Blak Friday Limited | Information & communication | 2025 |
| Blake & Brew Ltd | Hospitality | 2025 |
| Blake Riley Consulting Ltd | Professional & scientific | 2024 |
| Blake View Media Limited | Information & communication | 2025 |
| Blake's Sparkle and Shine Ltd | Administrative & support | 2025 |
| Blakedew 840 Limited | Financial services | 2020 |
| Blakedew 841 Limited | Financial services | 2020 |
| Blakepine Limited | Wholesale & retail | 2017 |
| Blakesley Estates (Duporth) Ltd | Construction | 2021 |
| Blakestones Entertainment Ltd | Arts & recreation | 2019 |
| Blakk-It Ltd | Information & communication | 2015 |
| Blakstar Ltd | Hospitality | 2020 |
| Blanc & Mist Limited | Wholesale & retail | 2025 |
| Blanc Lemont Ltd | Professional & scientific | 2025 |
| Blanc Velvet Co. Limited | Information & communication | 2025 |
| Blanco Digital Ltd | Other services | 2015 |
| Blancops Ltd | Wholesale & retail | 2025 |
| Blandford Laundry Services Ltd | Other services | 2018 |
| Blandon Parking Company Ltd | Households | 2022 |
| Blandon Way Management Limited | Households | 1979 |
| Blank Page Developments Limited | Construction | 2017 |
| Blankubedi Ltd | Transport & logistics | 2017 |
| Blaqgold International Ltd | Construction | 2019 |
| Blask Agencr Limited | Wholesale & retail | 2025 |
| Blasus Ltd | Hospitality | 2020 |
| Blaustein Limited | Administrative & support | 2011 |
| Blayer Limited | Wholesale & retail | 2021 |
| Blaze Medical Limited | Wholesale & retail | 2021 |
| Blaze Mist Ltd | Wholesale & retail | 2025 |
| Blazing Lounge Limited | Hospitality | 2025 |
| Blazing Media Ltd | Information & communication | 2025 |
| Blazing Star Ltd | Wholesale & retail | 2023 |
| Blbn Ltd | Real estate | 2022 |
| BLC (Wales) Ltd | Education | 2010 |
| BLC Fashion Ltd | Wholesale & retail | 2025 |
| BLC Solutions U.K. Ltd | Information & communication | 2020 |
| Bledisloe Estate Holdings Limited | Real estate | 2020 |
| Bleg Dori Ltd | Wholesale & retail | 2024 |
| Blekking Strategic Communications Limited | Professional & scientific | 2018 |
| Blend Cardiff Limited | Hospitality | 2021 |
| Blend Telecom Limited | Information & communication | 2018 |
| Blend Unit Ltd | Other services | 2026 |
| Blendable Labs Ltd | Wholesale & retail | 2025 |
| Blenman Logistics Ltd | Transport & logistics | 2023 |
| Blessed & Boujie Ltd | Hospitality | 2026 |
| Blessed PAT Stores Limited | Health & social care | 2021 |
| Blethin (Llandaff) Management Company Limited | Hospitality | 1983 |
| Bleu Dexx Limited | Information & communication | 2025 |
| Bleudoor Ltd | Other services | 2025 |
| Bleuribbon Hair Group Limited | Manufacturing | 2017 |
| Blevlez Services Ltd | Administrative & support | 2022 |
| Blew Room Hair Limited | Administrative & support | 2010 |
| BLH Estates Limited | Real estate | 2013 |
| Blietamin Supplies Ltd | Manufacturing | 2025 |
| Blighty Apps Ltd | Wholesale & retail | 2019 |
| Blighty PUB Company Limited | Hospitality | 2021 |
| Blind Baseball Cymru CIC | Education | 2023 |
| Blind Dates Ltd | Information & communication | 2021 |
| Blindfaith.Online Ltd | Wholesale & retail | 2021 |
| Blindspot Consulting Ltd | Information & communication | 2018 |
| Blindspot Ltd | Information & communication | 2018 |
| Blingo Competitions Ltd | Wholesale & retail | 2021 |
| Blingor Ltd | Wholesale & retail | 2024 |
| Blink Blink Cardiff Ltd | Other services | 2020 |
| Blink Design Services Limited | Information & communication | 2017 |
| Blinkie Music Production Limited | Information & communication | 2016 |