"Blocked"

I'd like some input from those of you who are familiar with the
working ATC environment.

In a situation where a transmission to/from ATC is "interfered with"
by a second transmission I've always believed that it is useful to let
all parties know that the tranmission was blocked, even if I suspect
that I'm not the intended receiver. In order to make sure that the
transmission did not go through despite the interference, I usually
wait a few seconds to see if there is an answer. Often there is and I
just keep quiet.

On Sunday I was flying (VFR) in the Ontario, CA Class C, talking to
ATC in the northeast sector. Things were not as zoo-ey (a technical
term, sorry) as they can be in Socal space, in fact there was not alot
of congestion on the freq. However there were 2 transmissions that
occured at the same time making the first part unintelligable. From
the last few words (from ATC) I strongly suspected the transmission
was intended for me. After waiting a few seconds, I transmitted
"Blocked". The angry response from ATC was "who said 'blocked'?". I
repled that I did and he said "don't do that". He later had time to
explain that this often blocks another of his transmissions and that
it can interfere with a second frequency that he may be using or that
the transmission is not "blocked" at all. He said it is his preference
for pilots not to do this but if they do, they should also give their
N number (e.g. "blocked, N12345").

So here's my question: it's my impression that such a short
transmission almost never causes confusion or interference. Rather it
quickly clears up the situation enabling other aircraft to talk,
freeing up the frequency. So you controllers out the which is it?
Does this help or hurt the situation?