I would very much doubt that anyone bothered to establish that. This strain was engineered purely for molecular biology purposes and is not virulent. Most likely any serotype determinants of the parental strain were also lost over last 30 years of laboratory propagation. 

As you can try to get the BL 21 strain from Dept of Mol. Bio, Syngene International Pvt. Ltd, Bangalore, on request

Please take a look at this article

Gene. 2006 Dec 15;384:113-9. Epub 2006 Jul 31.

Escherichia coli BL21(DE3) chromosome contains a group II capsular gene cluster.

Andreishcheva EN1, Vann WF.

Author information

1Laboratory of Bacterial Polysaccharides, Center for Biologics Evaluation and Research, Building 29, Room 103, 8800 Rockville Pike, Bethesda, MD 20892, USA.

Abstract

During our study of de novo synthesis of Escherichia coli K1 capsular polysaccharides, we found that E. coli BL21(DE3) has a capsular gene cluster, similar to those of group II capsular E. coli strains. Analysis of the nucleotide sequence of the E. coli BL21(DE3) gene cluster showed homologues to all group II regions 1 and 3 genes and the presence of an IS1 element in one of the region 2 ORFs, which likely prevents capsule expression. Complementation analysis showed that region 1 and 3 genes encode functional proteins that are sufficient for the export of newly synthesized polysaccharide. The gene products of Bl21(DE3) kpsC and kpsS supported in vitro de novo synthesis of K1 polysaccharide when co-expressed with K1 NeuE and NeuS. Sequence homology between BL21(DE3) region 2 open reading frames and capsule-related genes in other bacteria such as Haemophilus influenzae serotype b, suggests that the encapsulated ancestor of BL21(DE3) may have produced a ribose/ribitol-phosphate containing polysaccharide.

O7 is the serotype. Most phylogenetic papers have it

Aside from the defect in the O7 O-antigen operon mentioned above, BL21 strains are also defective in biosynthesis of the R1 O-antigen core oligosaccharide. The Gal operon has a large deletion (galM-ybhC) and R1 O-antigen core operon has an IS1 insertion in LPS 1,2-glucosyltransferase, resulting in an exposed truncated outer core (rough mutant). These observations are based on comparisons of genbank reference sequences.