Two Levels

The famous question: why no beracha on sending mishloach manos?
Not such an easy one to answer.  Traditionally we are supposed to keep the other Purim mitzvos in mind while listening to the blessings before the megilla but why shouldn't there be a separate blessing for the others? 
For the seudah and matanos l'evyonim, the answers are relatively easy.  A meal on Purim lacks anything particularly distinctive to demand its own beracha.  As for giving to the poor, there is the famous psak of the Rashba that explains why tzedakah doesn't get a blessing - any mivtzah that depends on another person for its completion cannot have a blessing since the other person might refuse the charitable donation making the blessing a vain one.
In truth, many try to apply that reasoning to mishloach manos since it is similar in some ways to charity.  However, many poskim hold that it is sufficiently different to make the Rashba's psak not relevant to this case. 
For one thing, the purpose of mishloach manos is not to give a gift to the needy but to spread love and brotherhood amongst the Jewish people.  Some say that the purpose of the mitzvah is to counter Haman's words in the megillah, that we are a scattered and divided people.  By sending gifts to one another we provde this untrue.
If this is so, then the mitzvah is in the sending, not the receiving of the gift.  Slam the door on the messenger all you want, his having sent it in an attempted stab at friendship fulfills his requirements.  Thus one should be able to make the beracha if that's the case.
Now, there is the position of the Rambam from hilchos Berachos in which he notes that while ben adam l'makom commandments generally get blessings, ben adam l'chavero ones don't.  What the reason for that is remains to be explained.
One explanation is that there is little in most ben adam lchavero commandments to distinguish them as specifically Jewish.  Visiting the sick, burying the dead, helping the poor, and so on all can be found amongst all the nations of the world.  Jews performing these activites may do so l'shem mitzvah instead of simply because "it's the right thing to do" but to the outside observer, there is no difference between what we do and what eveyrone else do in these matters.  Hence some poskim hold that blessings are reserved for distinctively Jewish activities that are generally classed as ben adam l'makom.  So this might explain why mishloach manos might not get a beracha.
The Seridei Eish (hat tip to Nishma for pointing this out to me) notes a different, fascinating answer.  Generally in Jewish law we have a principle that it is a greater thing to do something one is commanded to do than to do it from a voluntary position.  Men are commanded to wave a lulav on Sukkos.  Women can but don't have a command to do so.  Men are therefore considered to have the greater reward, possibly because once something becomes a command the yetzer hara steps in to try and prevent the action but that's a separate topic.
However, the Seridei Eish implies that while this applies to ben adam l'makom commandments, it doesn't necessarily apply to ben adam l'chavero.  Looking specifically at mishloach manos, he repeats the concept that the mitzvah has the purpose of spreading love and brotherhood amongst the Jewish people.  Obviously sending gifts could have been commanded but the real value in a gift is its being offered freely instead of through coercion.
(As a side note, this could explain why the Mishkan was built almost entirely from free-will gifts instead of a mandatory tax)
As a result, the Seridei Eish holds that by applying a beracha to mishloach manos would remove the very purpose of the mitzvah, hence no blessing.
But there's a deeper concept I want to mention that emerges from this.
As written above there are two levels of mitzvos.  Which are the more basic?  One would have to say, based on a review of the Tanach, that ben adam l'chavero, the not-so-glamorous, not-so-specifically-Jewish commandments are the basic level of observance.  Consider the famour drash as to why the generation of the Flood was destroyed while the generation of the Tower of Bavel was only dispersed.  Consider how the prophets repeatedly emphasize social ills as the cause of the destruction of the Temple and our subsequent exiles.  After all, of the three cardinal sins that destroyed the First Temple, only one is ben adam l'makom.  The main sin that destroyed the Second Temple is entirely ben adam l'chavero.
Thus if one were to look at the two levels in a structural fashion, ben adam l'makom forms the second floor while ben adam l'chavero forms the first. 
And while the prophets make clear that God can build a structure by first spreading out the roof and then filling in underneath, human beings can only put something up from the ground level first.  A properly functioning Jewish system must be based on a strong dedication to ben adam l'chavero that then leads to the level of ben adam l'makom.  Anything else is like trying to put up the second storey while the first is yet incompletely.  The whole structure then collapses into a ruin.
Perhaps this is why there have been so much trouble reported out of the Torah observant community in the last while.  For a couple of generations now, the emphasis in the "most kosher" circles has been almost entirely on ben adam l'makom, many times openly at the expense of ben adam l'chavero.  Human dignity is shoved aside in a crazy delusion that somehow God doesn't care how we treat each other, just so long as we shokl hard enough when we daven and our black hats have big enough brims.  Worrying about what our Father in Heaven thinks of the length of our tzitzis while we shove our way rudely past our fellow man is completely in the wrong direction.
This Purim let us all take a step back.  The prophet quotes God as saying that given the choice between forming a functioning society and worshipping Him, he would choose the former because a functioning society will ultimately lead to people approaching Him.  Sole worship of Him will ultimately fail because the dysfunctional society underneath will take everything down with it.  Are you kosher enough?  Don't worry, God forgives.  Did you steal or cheat your fellow man?  God's not so crazy about that, or so the prophets have told us.
We must restore the proper direction of Judaism this Purim by emphasizing the mishloach manos in our lives.  One day a year to show communal fraternity is nice but really, mishloach manos are something you can do all year around.  And maybe that's another reason there's no blessing on the mitzvah.  Something we must always be involved with doesn't require a blessing because there's no start and certainly no end to the efforts.