athonite
As far as I am aware, German law does not dictate a specified interval between blood donations and flying. The decision as to whether (or when) a pilot is fit to fly after having donated blood is basically left to the pilot's own judgement, however medical advice must be sought when there is doubt. Commercial operators must include provisions in their OMs. From what I have heard, some OMs forbid all blood donations, except in cases where near relatives suffer from life threatening conditions and require a blood transfusion. According to posts on German forums, some pilots seem to believe there was a waiting period of four weeks; my impression is this goes into the category of urban legend.
Under section 1, subsection 3 of the German Aviation Regulation (Luftverkehrs-Ordnung, LuftVO) "no person shall act as a pilot [...] while his function as pilot [...] is impaired [...] due to mental or physical impairments." (English translation from ENR 1.1 of the German AIP). As this provision remains rather general, it leaves much room for interpretation. Apparently, there is no precedent with regard to blood donations and the German civil aviation authority, LBA, has not published an administrative opinion. Were it not for the European Aircrew Regulation (see next paragraph), this provision of the Aviation Regulation alone would, in my opinion, require some waiting after blood donations.
The provisions of JAR-FCL 3.040, which have been incorporated into national law in Germany, have now made their way into Part-MED of the amended Aircrew Regulation [Commission Regulation (EU) No 1178/2011], which will apply from 8 April 2013. Under JAR-FCL 3.040(a) and MED.A.020(a)(1) of the Aircrew Regulation, licence holders shall not exercise the privileges of their licences when they "are aware of any decrease in their medical fitness which might render them unable to safely exercise those privileges". In my opinion, this provision does dictate a period of abstinence from flying after blood donations, but the length of such period must be determined based on the circumstances of each individual case, taking into account the instructions and information usually given to blood donors. The relevant AMC1 MED.A.020 calls for (aero-) medical advice when there is doubt.
For commercial operations, EU-OPS 1.085(d)(3), EU-OPS 1.1045 and Appendix 1 to EU-OPS 1.1045 apply. According to OPS 1.085(d)(3), a crew member shall not perform duties on an aeroplane "following blood donation except when a reasonable time period has elapsed". The operations manuals must contain "guidance to crew members concerning health including [...] Blood donation" (EU-OPS 1.1045 and Appendix 1 to EU-OPS 1.1045).