Susie...yes there should be a blank line between a centered heading and the key to a tactile graphic. As always there is an exception...the pages you reference, 3-35 through 3-36 are examples of a graphic with a multiple page key. That spins things off in an entirely different direction. On these pages, (because there are multiple key pages to the graphic) the graphic is on the left and the key pages are on the right. Because the centered heading (or title) of the map goes with the map on the left (even page) and it is repeated on the right (odd page) the centered heading or graphic title now becomes a "repeated heading" and as per 5.3.1 a blank line is not required with repeated centered headings. Unless you are specifically working on a graphic with multiple page keys, where the above situation would apply, do not use this as an example of whether or not to leave a blank line between the heading and the "Key:" statement. Revisit the chart included in 5.3.5 which outlines the use of blank lines with tactile graphics in situations where you are using a running head and situations where you are not using a running head within the transcription.
Section 188.8.131.52 does state that a blank line should be left between a centered graphic title and the key when it is the first page, or only page of a graphic. If page 1 of a graphic contains the centered heading then the key...you would need a blank line because you always have a blank line before and after a centered head. The exceptions on pages 3-35 through 3-37 are because these are repeated headings found on odd pages. You don't leave a blank line after repeated headings unless the repeated heading is being followed by the graphic itself...and you always have to leave a blank line before and after a graphic.
If you want to send me an example of what you're doing, I'd be happy to review and let you know if you've applied the blank line concept correctly.
Good luck!!! Diane