"Elderberry Toxicity

European black elderberry is known to contain a compound, sambunigrin, which is a cyanogenic glycoside that breaks down to hydrogen cyanide after ingestion and can cause gastrointestinal disorders such as nausea, vomiting, weakness, and dizziness when ingested in large quantities. The leaves, seeds, bark, and unripe berries of black elderberry are known to contain relatively high concentrations of sambunigrin, while ripe berries contain lower concentrations (Senica, et al., 2016).

Heating black elderberries while processing them into final food products such as juice, jelly, liqueur, or tea, has been shown to reduce the levels of sambunigrin by 44% to 96%, depending on the amount of heating. The resulting levels are deemed safe to consume in normally accepted serving sizes for these products, as the human body has some natural capacity to break down a limited concentration of cyanide (Senica, et al., 2016).

Research on American elderberries conducted at the University of Missouri found only low levels of cyanide in fresh berries, and cyanide levels in juice & seeds were lower than those in stems & green berries (Thomas, 2019).

The level of sambunigrin in blue elderberry is unknown. Some individuals of Native American descent have reported eating raw ripe berries, likely in small quantities, with no ill effects, even as children, suggesting that concentrations may be low, but levels of sambunigrin in blue elderberry have not been confirmed by laboratory analysis. However, when processing ripe berries and flowers, it is a best practice to remove all stems and leaves from the finished product to completely eliminate plant parts highest in cyanogenic glycoside."

I am unable to find any resource that indicates the sap is a skin irritant, but would urge caution to make sure that you are not inadvertently harmed.  Good luck!