See Bluebook Rule 21.9(iv)
COM documents are proposed legislation and other Commission communications to other European institutions and the Commission's preparatory papers. SEC documents are internal documetns conerning the operations of Commission documents.
Example:
Proposal for a European Parliament and Council Directive amending Council Directive 91/308/EEC of 10 June 1991 on Prevention of the Use of the Financial System for the Purpose of Money Laundering, at 3, COM (1999) 352 final (July 14, 1999), 2000 O.J. (C 177) 42.
"at 3" represents the pinpoint cite
"352" is the document number
Although the Bluebook doesn't require a parallel citation to the Official Journal, it is useful for research purposes to remember that all preparatory acts are published in the Official Journal "C" Series and a citation to the OJ is quick and convenient way retrieve the document on EUR-LEX, Westlaw or Lexis. See "Citing to the OJ" under the Official Journal sub-tab under the EUR-LEX tab.