Question

Can git blame show the date that each line was merged to the current branch rather than the date it was committed?

Aside: in some trial and error I notice this oddity:

 $ git blame --merges
 usage: git blame [options] [rev-opts] [rev] [--] file

 [rev-opts] are documented in git-rev-list(1)
 ...

 $ git help rev-list | grep -F -e --merges
 [ --merges ]
 --merges
 --min-parents=2 is the same as --merges.
--max-parents=0 gives all

Answer 1

1) Not exactly. Git does not record when a commit entered a particular branch (or what the "ours" branch was called during a merge). If you follow a topic branch workflow in which an integrator merges each branch into master, then following the first parent of each merge will show the commits directly on master.

You can see this in action in git.git, which follows such a workflow, by doing "git log --first-parent", which shows only the commits created directly on master (mostly merges of topics, with a few trivial fixups
interspersed).

Similarly, you should be able to do "git blame --first-parent foo.c" to pass blame only along the first-parent lines. Unfortunately, while "blame" uses the regular revision code to parse its options, it does its
own traversal and does not respect each option. However, the patch to teach it about --first-parent is pretty trivial:

diff --git a/builtin/blame.c b/builtin/blame.c
index 57a487e..0fb67af 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -1199,6 +1199,8 @@ static int num_scapegoats(struct rev_info *revs, struct commit *commit)
 {
 int cnt;
 struct commit_list *l = first_scapegoat(revs, commit);
+ if (revs->first_parent_only)
+ return l ? 1 : 0;
 for (cnt = 0; l; l = l->next)
 cnt++;
 return cnt;

(though I suspect it would interact oddly with the "--reverse" option, and we would want to either declare them mutually exclusive or figure out some sane semantics).
2) coming to problem of merges your problem is not the presence of "--merges" here, but that you forgot
the necessary "file" argument. Try "git blame --merges foo.c".

However, this suffers from the same problem as --first-parent, in that it is accepted but not respected. Doing so would not be impossible, but it is a little more than the two-liner above.