I have recently had a bloody nose during shacharis, and a drop of blood got on the inner part of my tefillin retzuos hanging down from my Shel Rosh. Do I now need to replace my retzuos because the (obviously non-kosher) blood got on them?
Answer:
You don't have to worry about this, and certainly not to replace the retzu'os because of this.
Best wishes.
Sources:
There is no obligation to paint the reverse side of the tefillin straps black.
There is an opinion that the edges of the retzuos should be painted black (Keses Ha-Sofer 23:2). However, this opinion is not accepted in halachic practice (see for example, Mishnah Berurah 33:24 quoting Pri Megadim in Eshel Avraham 33:7).
Therefore, the fact that a drop of blood fell on the reverse side of the straps will not constitute a problem.
Best wishes.
