Thanks for the question Marty!
Assuming that the back side of a TG (even page) is the last page of the volume, one would not apply either a print or braille page number. This assumes that “The End” will fit on the page with the tactile.
In the case where we are not at the end of a volume agencies may treat it somewhat differently. For example, if a graphic is on a100, the back shows b100, and the next odd page would be c100.
Grounding for these in GSTG and BF follows:
Since this question pertains to the page numbering of tactile graphics pages, we refer to GSTG Unit 5, Section 5.12.1 for interpoint braille, where on pages 5-24 and 5-25 the images show the preferred method of page numbering.
However, as you might acknowledge there not simply one answer that applies to all situations.
In particular, if the TG has a key page it would normally be bound facing a graphic, which may result in the need for an extra blank page. In addition, if there are multiple key pages, GSTG §5.12.4 shows images to indicate the page numbering for a single-sided volume (see page 5-27). See also page 5-30 for complex diagrams with multiple parts and key pages.
We also want to point out that in Braille Formats: Principles of Print-to-Braille, Appendix E.1.1 states that these are recommendations only, with the intent of providing a consistent approach. This allows agencies to set their own standards. (And our experts acknowledge that agencies do differ in their approach.)
All of that comes together to indicate that we should follow guidance in GSTG and BF if at all possible. If agencies decide to deviate from this, it should be done with consistency.