As stated in the Guidelines for Grant Applicants, page 20, section 2.1.2, please refer to Annex IV to the Standard Grant Contract for specific rules regarding procurement procedures, particularly to sections 2.1, 2.2 and 2.3.
You can find this annex (inside Annex F: Standard Grant Contract, zipped file), alongside other documents on the programme website, particularly in the following link: http://www.blacksea-cbc.net/index.php/eng/Documents/Guidelines-for-applicants
You will also find in this link, Annex G: Eligibility of expenditure by budget lines. Please refer to the following sections: 3. Equipment and supplies and 5. Subcontracted services (external expertise and services) for the rules on nationality and origin.
Please also refer to the Practical Guide to contract procedures for EU external Actions (PRAG) for a comprehensive definition, specific rules and exceptions of the rules on nationality and origin. (Sections 2.3.1 and 2.3.2) You can find the PRAG in the following link: http://ec.europa.eu/europeaid/work/procedures/implementation/index_en.htm