We don't honestly know.

The majority of our information about pagan Scandinavian practices comes from archaeological sources (which necessarily preclude any discussion of blasphemy, as they're sorely lacking in instructional manuals) or either foreign Christian or native post-Conversion literary sources, which aren't exactly keen on showing how the pagans worshipped in anything other than a bloodthirsty way. The niceties of the actual worship are usually glossed over or distorted quite a bit.

There is a relevant passage in the beginning of Eyrbyggjar Saga concerning the Icelandic settler Hrólfur Örnólfsson, called Þórólfur Mostrarskegg (Thor-Hrolf Bushy-beard). This man dedicates a lot of his land to Thor and in doing so prohibits his people and neighbours from spilling blood or squat down and do the deed in these areas. A tiny island poetically named Shit Rock was made into the designated toilet.

Some neighbours coming to ting in Þórólfurs land negated to follow this rule and wanted to shit where ever they damned well pleased. This resulted in Þórólfurs people attacking them and lives lost on both sides whereafter the land, due to the spilled blood, was no longer considered holy. The two groups were very hostile to each other for the rest of Eyrbyggjar Saga.

EDIT: Formatting

Unfortunately I am travelling and don´t find Islendingabok online. However if I remember correctly, Althingi accepted laws on blasphemy against the old gods the year before Iceland officially converted. It is not a good source, since it is written down more then a century later, but I think most academics dont reject his telling about the conversion, but dont take it overly literally. If true however, it indicates that prior to the fierce struggle over conversion, there was no law against blasphemy.