The available records clearly show that Jesus of Nazareth was crucified for political and religious crimes by the Roman govenment of the period. Thus, historians are perfectly within their craft to state so.
The questions of his "dying for our sins", or "the Resurrection" are only testified to as fact in the Bible and by those faithful to it. While fine for them, this does not meet the standards of proof required for history's sake. That is hardly a blasphemous statement, as truth cannot be blasphemous.
One can debate history or one can debate theology. One cannot debate both at once, as there are completely differing standards of proof for both. One does NOT, in all cases, invalidate the other.
Religious dogma CAN be useful to the science of history, as it can provide possibilities which can be investigated of unexplained events. History, likewise, can be of value to the religious, in telling the tales of religious figures as actual events, and their signifigance to the world at large.
No one, no historian, could deny, for example, the impact that Jesus had upon world history. They must, however, explain that impact by what is recorded as provable fact. Alas, faith, however strong, cannot meet this requirement.
Slinging the "blasphemer!" tag around too readily only burdens those seeking objective truth, and comes too close to the refusal to see, or look upon such truth. What can be so frightening about it, that the eyes must be shut?