You haven't significantly compromised strength by removing the last pin and the back of a drawer actually doesn't need to be that strong¹ so I don't think it's a big concern.

But obviously it is best not to lose a part of a joint (even if only so the effort to make it isn't wasted!) so I'll list some options for you.

Sticking with the Blums
For future drawers you can simply change the style/spacing of the dovetails and you're golden — no shallow pins there, zero loss of strength once the notches are milled.

If you just can't stand to do uneven pins you can always use Blum's own recommendation for the no-notch version of mounting, which is included in the PDF you linked to in the Question on page 11:

An even simpler option is don't dovetail the back — nobody ever sees them and nobody really cares but the woodworker LOL And they certainly aren't needed for strength on a drawer back — the backs of drawers typically see almost no forces exerted against them trying to push them off. As a result even the simplest option of all, butt joints, are more than adequate for the back of virtually any drawer if glued and pinned or reinforced with skinny dowels.

If a butt joint is unacceptable for some reason, you could use a housing joint (dado joint) instead, although you inherently lose a small amount of drawer volume this way since the back has to be brought forward slightly.

Select different slides
There are different makes of undermount drawer slides and not all require the same positioning. So another option if you must dovetail all four corners and you'd prefer to continue to do them exactly as done already is to simply pick a different style of slide.

¹ Traditionally both front and backs of drawer boxes were dovetailed because they were overbuilding for strength, as well as creating a joint that held together mechanically because they knew from experience that their glues sometimes failed. We don't really have that concern any more, unless you're using a protein glue of course!

² Bamboo skewers are great for this. Short lengths of bamboo dowel are extraordinarily strong and make a great alternative to pins if one wants to avoid metal fasteners for any reason (e.g. so the sides of the drawer remain safe to plane).

You haven't significantly compromised strength by removing the last pin and the back of a drawer actually doesn't need to be that strong¹ so I don't think it's a big concern.

But obviously it is best not to lose a part of a joint (even if only so the effort to make it isn't wasted!) so I'll list some options for you.

Sticking with the Blums
For future drawers you can simply change the style/spacing of the dovetails and you're golden — no shallow pins there, zero loss of strength once the notches are removed.

If you just can't stand to do uneven pins (they do look odd) you can always use Blum's own recommendation for the no-notch version of mounting, which is included in the PDF you linked to in the Question on page 11:

An even simpler option is don't dovetail the back — nobody ever sees them and nobody really cares but the woodworker LOL And they certainly aren't needed for strength on a drawer back — drawers typically see almost no forces exerted rearwards trying to push the back off. As a result even the simplest option of all, butt joints, are more than adequate for the back of virtually any drawer if glued and pinned or reinforced with skinny dowels².

If a butt joint is unacceptable for some reason, you could use a housing joint (dado joint) instead, although you inherently lose a small amount of drawer volume this way since the back has to be brought forward slightly.

Select different slides
There are different makes of undermount drawer slides and not all require the same positioning. So another option if you must dovetail all four corners and you'd prefer to continue to do them exactly as done already is to simply pick a different style of slide.

¹ Traditionally both fronts and backs of drawer boxes were dovetailed because they were overbuilding for strength, as well as creating a joint that held together mechanically because they knew from experience that their glues sometimes failed. We don't really have that concern any more, unless you're using a protein glue of course!

² Bamboo skewers are great for this. Short lengths of bamboo dowel are extraordinarily strong and make a great alternative to pins if one wants to avoid metal fasteners for any reason (e.g. so the sides of the drawer remain safe to plane).